You'll have to decompile the binary model (MDL/MDX) into an ASCII version using either MDLEdit or MDLOps. This can be then be imported by KOTORMax for Max/GMax, or KOTORBlender for Blender. There's no native solution for Maya. You'd have to import it into either one of those first and then export it in some other format to get into Maya, but be aware that this is basically unworkable for skinned meshes while preserving their weights, due to the way KOTOR models work.
You can modify heads, bodies, etc., or make completely new ones. This was somewhat limited in the past due to the tools available, but has changed significantly in the last 12 months.