Backing up a little:

For the most part, editing things in Jazz 2 is done by writing code. For users, such as you, the code you write must be in the language called AngelScript.

There is not a graphical tool you can use to import a 3D model or something which automatically creates a character. Some things can be done in AngelScript pretty simply, because they don't have a lot of steps or because code libraries have been developed to make them easier. Writing a custom character is not so easy because it has a few different pieces.

The first and most important step is getting your sprites into the game. Use some variation on this angelscript here. You will also need a .j2a file with the animations in it. Use these Python scripts.

Once you've gotten your sprites in the game, and only once you've done that, it makes sense to start worrying about unique abilities. But the sprites are the first step.