I assume you're talking about adding custom models to the species selection screen for Jaden in Single Player? It can be done, but it's not easy. Unless you're fairly knowledgeable in manipulating code, I wouldn't advise trying to do it. If you're still up to the challenge, there is an in-depth, two-part tutorial explaining how to do it. You can find the tutorial here (part 1) (
http://www.map-review.com/index.php?page=tutorial_view&id=7) and here (part 2) (
http://www.map-review.com/index.php?page=tutorial_view&id=8). Note that you only really need part 1 to get a character in the game, part 2 just takes it a step further by allowing you to add it to the species selection screen.
If you're talking about playing as some other character included in the game in Single Player mode, (such as Boba Fett or a Reborn), all you have to do there is load a level, bring up the console screen (hit the ~ key to do that) and type in "playermodel whatever." For example, if I wanted to be a Reborn, I would type in:
playermodel reborn
If you're just talking about using custom models/skins in Multiplayer, all you have to do there is download the file, open the file using some sort of zip program like WinZip or WinRar, extract the pk3 file to Jedi Academy's base folder, and it should now be available at the character select screen for MP.
Hope that helps!