What are you asking exactly? Are you talking about porting textures from one character to another? Or previewing with different textures. And how does BrawlBox and your game crash from it? You may be doing something wrong.
Here is what I am asking.
In BrawlBox, on a character, can I go Edit > New > Texture, and add a new texture to a character, after which you place the texture in the character file. For example, Mario doesn't have a "Mask" texture like Sonic does, would it be possible to create a new mask texture from scratch, for Mario, then make a new texture slot in his FitChar##.pac and add it in? If so, there is a whole host of things I can do.