A mod where 1 character gets swapped can be done. However unlike L4D2 and Gmod, Bo3 can only load 1 mod at a time which is likely why it hasn't been done.

makes sense, idk why bo3 cant load more then 1 mod, seems sorta dumb
if you dont mind me asking, but how exactly do i use this mod along with custom maps?

Click "mods" in the main menu and load the mod. Once you've done that, you should see a yellow "mod loaded: platformer_pack" text in the top right to let you know it's active.