Possible player count increase through modding?

Would a player count increase for co-op, say from 2 to 3 or 4 players be possible with mods? I have 2 other friends wanting to get the game but there would always be someone left out which absolutely sucks. We've been playing 3-4 player games for so long but having to cut someone out of the group just makes everyone feel bad about even playing.
 
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