hmmm. i think it would be possible.
i hex-edited savegames to make characters jedi. so i am pretty sure you can change character classes with other tools as well.
here is a thread where this is somewhat discussed, though it involves the use of the gff editor and the erf editor, and still focuses on editing a savegame.
here's another thread, but once again does not involve kotor tool:
i am not sure where your npc class info is stored otherwise, that may allow you to edit it with kotor tool. i will look into it.