I have Gmax 12, and i'm wondering if it's possible that exists a plugin for NIF files... Where can i found it?
I'm quite sure that there is none. There's just a plugin for Discreet 3D Studio MAX R3.1 and Milkshape.
I have answered this question before - as a fellow GMax user, I want there to be a plugin, but there is none. There is, however, a place on the gmax website (discrete website) where you can petition the makers to create pulgins for various games. I know of three or four users of this site who have made such an appeal, obviously more must be needed!