You can via loophole. You have to create a new multiplayer game and go into the character creation screen, then open up 3 other instances of the game with new profiles and have them join your game. You should be able to customize all of them. I customized two characters with that method.

Here's the link for more precise info: http://steamcommunity.com/sharedfiles/filedetails/?id=769494798

All credit goes to user Strazyplus who figured it out.