Technically, it's possible. Sure. You wind up keeping the mesh and the textures and tossing the skeleton and animations, using existing ones from NWN and re-weighting the skin mesh yourself. For instance, I did this with my Automotoi, Arabian Nights skeletons and the rhinoceros I contributed to Draygoth's mount pack. It can take quite bit of work, though.
As a general rule, any mesh that you can load into 3DS Max/GMax can be exported into NWN.
How well it runs and looks, though, is usually dependent on how much care went into the conversion and how good the model was to begin with.
Edit: Also, NWN2 models tend to lean real heavily on their normal maps. Most of the time I don't think they'd look that great converted to NWN 1.