The catch with new player models would be that the server, as well as the client, would require the .pk3 files with them included.
As for doing the whole getting them into the game factor, just use something like pakscape or winrar or winzip to open up the player models .pk3 (pak0 or pak1, I forget which), then unpackage the animation.cfg file, animate the model based on that file, and blah blah blah. If ya don't know how to get a model into Q3, go check by Polycount, that'll get you set for it.
Mostly, as for learning how to get it implemented, my suggestion would be to do what Lixus said and replace the old ones, given that most of the time the br mode is in effect, and in that mode you have to choose from the original set of player models.
Hopefully these new models of yours turn out nicely, since if I ever do anything in Western Q3 (movie-wise) ever again, I'd like new models!
(note: if message seemed weird and irrational - I got a flu, so my brain isn't functioning 100%)