Obviously, this part of the game isn't finished yet, but I have confirmed that it is relatively easy to add your own player sprites and tokens to the game, and as far as I can tell it doesn't break anything.
As a test, I created a quick pallet-swapped green version of the playable Mantis character PNGs (I just added a 2 to the first part of the file names) and it worked just fine.
Depending on how good you are at pixel art, you could probably insert anything you want in this manner, as long as the PNG dimensions are the same and you use the same file naming conventions.

The same goes for tokens. I'm currently in the process of adding a bunch of images from cheesy sword and sorcery movies for mine. As long as you give them different file names, it doesn't seem to have any sort of negative impact on the game.

