You're probably better off asking the makers of the mods to do it, but you can open the true co-op main.lua file and scroll down a bit and there should be a bunch of stuff in there about adding a character to the selection menu, basically you make a table with the characters info (health, starting items, etc) and then there's an 'addtowheel' function that adds the character to the menu that pops up when someone joins.

You will need to add some extra graphics for the character though, you can inspect one of my character mods to get an idea.