Well, modifying the existing model seems a little pointless to me.  You would first have to find a program that can do that, and after six months of searching through FS sites I haven't come across a single program like that.  Even if you do manage to convert the file, you will have to reanimate, rename and retexture the model.  So your best bet is to just start from scratch.  At least, that's my take on the matter.