well, i'm sure you could go and make the models in XSI or Maya or something equivalent, however, i wouldn't know how you could go and force those models on a particular map. players interested enough could download the model files as a game modification and they'd be able to see them in game, but only they would. it's sort of like the custom weapon skins you find on places like fpsbanana.
doing something like this would definately be a client-side mod. I don't think it'd be possible to do this as a server side mod.