Thanks for the recon FrieD!
I am a dunce and didn't even look past the milkshape site archives... doh!
Anyway, to answer you Hersh, I do not think you need to register the information with the Tribes Engine on a "registeruserobjects" list for player models and such, but I could be wrong.
I looked at the Screaming Fist mod and TRPG mod files to check this (though I am still looking) and have not found anywhere that this needs to be registered with the registeruserobjects file.
I remember reading a site long ago that said some thing about this... so I will try to find that again! *edit* I found the site but it is not much help in the ways of model additions... it says to rename them to replace old models.
Peace.
*edit*
If I were you Hersh, I would just name them what I wanted, add them, then make an item that references them as the model file... startup trpg and see if you can give it to yourself... and see if it looks good in game.