It is possible to use prestige classes that are not hard coded into NeverWinter Nights. It can be challenging, depending on what exactly you want to do but there are a large variety that do not require any character modifications. However, as these classes are not a part of the NWN engine, you have to play them out and rely on roleplay to achieve your required results.
Some prestige classes from the accessory books require DM intervention to fully achieve the required effect. If you approach a DM with your plans and a possible work-around, the DM may be happy to accommodate your request. You would stand a better chance if you have already created a background story and have already shown good roleplaying skills on that server.
Examples of Non-NWN Prestige ClassesEdit
Here's a small sample of the classes with suggestions on how to create a believable character.
Divine Oracle (Defenders of the Faith)Edit
Often depicted as madmen who "talk to God", those that choose to play as a Divine Oracle need to portray themselves as slightly unhinged because they are often not believed by others. They can often be told of past and future events by their God. All Divine Oracles are spellcasters and most were clerics or druids.
Requirements According to the book (Defenders of the Faith) you must have 10 ranks in Scry. As this is not a skill in the NWN engine, you might want to substitute another (perhaps lore as that's often used to replace such things on NWN servers) or leave the required skill points unused and "use" them as if you had the ability to use the "scry" skill.
Class Skills Divine Oracles are proficient with all simple weapons but are not proficient with shields or armour but are allowed these proficiencies if granted from their previous class. Because of their ability to use spellcasting, they should have a class that uses it or have skills in Use Magical Device to compensate and depending on their class. You should also try to obtain uncanny dodge as this is also an ability of Divine Oracle from level 5. Look at the Divine Oracle page for full details of this class and possible workarounds.
Magelord (Lost Empires of Faerun)Edit
The magelord is an arcane spellcaster who studies the ancient traditions of his craft. They are known for extremly fast spellcasting. They sometimes hire themselves out as a spellslinger or a professional arcane duelist. The believe they have the right to take power and wealth from the world and to be granted respect from others. The art of the magelord is passed down from generation to generation as like martial arts or fencing. In reality he is just a bandit lord that wields his power for his own gain. The magelord doesn't give away his secrets easily.
To become a magelord, you must seek one out and persuade him to teach you the secret ways of using magic in their own style. According to the book (Lost Empires of Faerun), you are required to spend at least 10 days in that wizzard's company and pay at least 10,000gp or offer a service to the equivalent value. This creates an ideal roleplaying situation which could be played out via a series of one-on-one DM events. Another possibility to achieve magelord status without a mentor is to obtain rare tomes and treatises costnig at least 10,000gp and spend 1 month learning the techniques contained within them. Wizards, wizard/rogues and wizard/assassins make the best magelords but wizard/blackguards can also be good. However, wizard/fighters are very rare as magelords. Intelligence is the essential ability for this class but constitution is a factor as they tend to live in uncivilised lands. See the Magelord page for further information.