There is no reason why players coming from AD&D shouldn’t be able to use the alignments that they are use to. In the hero system this would be a Psychological Disadvantage. The exact modifiers would be up to the GM but I suggest the following.

Common – Always Taken: It is common for the character view the world around him using this “filter”.

Modifers:

Strong: For the character to truly strife to have his actions and decisions fall under the outline below then they should be forced to take Strong as a minimum modifier. While the character would strife to make decisions the follow the guidelines, he would see the need to adjust his decisions from time to time.

Or

Total: A character could upgrade his modifier to Total. This character would insure that all decisions and action follow the outline below. There would be no middle ground, while this doesn’t necessarily mean that he would be unmoving, it does mean that a strong debate would be necessary to change his mind.

The basic outline of the different alignments are as follows:

LAWFUL GOOD: Characters of this alignment believe that an orderly, strong society with a well-organized government can work to make life better for the majority of the people. To ensure the quality of life, laws must be created and obeyed. When people respect the laws and try to help one another, society as a whole prospers. Therefore, lawful good characters strive for those things that will bring the greatest benefit to the most people and cause the least harm. An honest and hard-working serf, a kindly and wise king, or a stern but forthright minister of justice are all examples of lawful good people.

NEUTRAL GOOD: These characters believe that a balance of forces is important, but that the concerns of law and chaos do not moderate the need for good. Since the universe is vast and contains many creatures striving for different goals, a determined pursuit of good will not upset the balance; it may even maintain it. If fostering good means supporting organized society, then that is what must be done. If good can only come about through the overthrow of existing social order, so be it. Social structure itself has no innate value to them. A baron who violates the orders of his king to destroy something he sees as evil is an example of a neutral good character.

CHAOTIC GOOD: Chaotic good characters are strong individualists marked by a streak of kindness and benevolence. They believe in all the virtues of goodness and right, but they have little use for laws and regulations. They have no use for people who "try to push folk around and tell them what to do." Their own moral compass guides their actions, which, although good, may not always be in perfect agreement with the rest of society. A brave frontiersman forever moving on as settlers follow in his wake is an example of a chaotic good character.

LAWFUL NEUTRAL: Order and organization are of paramount importance to characters of this alignment. They believe in a strong, well-ordered government, whether that government is a tyranny or benevolent democracy. The benefits of organization and regimentation outweigh any moral questions raised by their actions. An inquisitor determined to ferret out traitors at any cost or a soldier who never questions his orders is good examples of lawful neutral behavior.

TRUE NEUTRAL: True neutral characters believe in the ultimate balance of forces, and they refuse to see actions as either good or evil. Since the majority of people in the world make judgments, true neutral characters are extremely rare. True neutrals do their best to avoid siding with the forces of either good or evil, law or chaos. It is their duty to see that all of these forces remain in balanced contention. True neutral characters sometimes find themselves forced into rather peculiar alliances. To a great extent, they are compelled to side with the underdog in any given situation, sometimes even changing sides, as the previous loser becomes the winner. A true neutral druid might join the local barony to put down a tribe of evil gnolls, only to drop out or switch sides when the gnolls were brought to the brink of destruction. He would seek to prevent either side from becoming too powerful. Clearly, there are very few true neutral characters in the world.

CHAOTIC NEUTRAL: Chaotic neutral characters believe that there is no order to anything, including their own actions. With this as a guiding principle, they tend to follow whatever whim strikes them at the moment. Good and evil are irrelevant when making a decision. Chaotic neutral characters are extremely difficult to deal with. Such characters have been known to cheerfully and for no apparent purpose gamble away everything they have on the roll of a single die. They are almost totally unreliable. In fact, the only reliable thing about them is that they cannot be relied upon! This alignment is perhaps the most difficult to play. Lunatics and madmen tend toward chaotic neutral behavior.

LAWFUL EVIL: These characters believe in using society and its laws to benefit themselves, Structure and organization elevate those who deserve to rule as well as provide a clearly defined hierarchy between master and servant. To this end, lawful evil characters support laws and societies that protect their own concerns. If someone is hurt or suffers because of a law that benefits lawful evil characters, too bad. Lawful evil characters obey laws out of fear of punishment. Because they may be forced to honor an unfavorable contract or oath they have made, lawful evil characters are usually very careful about giving their word. Once given, they break their word only if they can find a way to do it legally, within the laws of the society. An iron-fisted tyrant and a devious, greedy merchant are examples of lawful evil beings.

NEUTRAL EVIL: Neutral evil characters are primarily concerned with themselves and their own advancement. They have no particular objection to working with others or, for that matter, going it on their own. Their only interest is in getting ahead. If there is a quick and easy way to gain a profit, whether it is legal, questionable, or obviously illegal, they take advantage of it. Although neutral evil characters do not have the every-man-for-himself attitude of chaotic characters, they have no qualms about betraying their friends and companions for personal gain. They typically base their allegiance on power and money, which makes them quite receptive to bribes. An unscrupulous mercenary, a common thief, and a double-crossing informer who betrays people to the authorities to protect and advance himself are typical examples of neutral evil characters.

CHAOTIC EVIL: These characters are the bane of all that is good and organized. Chaotic evil characters are motivated by the desire for personal gain and pleasure. They see absolutely nothing wrong with taking whatever they want by whatever means possible. Laws and governments are the tools of weaklings unable to fend for themselves. The strong have the right to take what they want, and the weak are there to be exploited. When chaotic evil characters band together, they are not motivated by a desire to cooperate, but rather to oppose powerful enemies. Such a group can be held together only by a strong leader capable of bullying his underlings into obedience. Since leadership is based on raw power, a leader is likely to be replaced at the first sign of weakness by anyone who can take his position away from him by any method. Bloodthirsty buccaneers and monsters of low Intelligence are fine examples of chaotic evil personalities.