I've been exploring the same issue.
I know how to replace skins in the game with downloaded skins, however the new model that I've downloaded (the Vader model) isn't quite the same as a skin. I can't figure it out. Nobody seems to know how
In the meantime, try this for any *skins* that you've downloaded... First of all, there is a big difference from a SKIN and a MODEL. Darth Vader is a model, because his polygon construction is different than any other character in the game (kinda like trying to make a cube from a sphere. You can't just "skin" it). Skins, on the other hand, are just like painting an already existing model, such as the Galak model or the Luke model with different textures.
Adding a skin into the game is simple.
-Just unzip the .pk3 containing the skin file.
-Change the name of the skin folder in your new unzipped folder \models\player\(skin name). Change whatever the name of that folder is to someone IN the GAME you want to replace (for example, I replaced Tavion with Darth Maul, so I changed the Darth_maul folder to tavion.)
-If there are any sounds for that character, just go into the folder sounds\player\(skin name) and change the name of that folder as well. Again, in my case I changed it to Tavion because that was the name of the character in the game I was replacing.
-Then, you zip the .pk3 back up (select all the folders in your unzipped directory, create a new file, etc.) and place it in your base directory.
I hope you get something out of this
I know the directions were kindof vague. I'm assuming you know how to convert from zip to pk3 (not too hard, really.)
Any questions e-mail me, MrTencza@aol.com