Alas, that is not viable - due to specifics of how the particular game was made, it would require substantial changes to the game, which would then have to be re-applied whenever an update is released. That's months of work.
The game seems to be doing alright so I'm sure that the developer will hire someone to do an official multiplayer implementation when they are able.
I helped with a slightly similar (has one player control a hero while the rest take control of enemies and try to kill them) game called Ghost Croquet though, if that's any help. Was made with online multiplayer in mind from start and has a level editor.