It would be fairly easy to create yourself, since it would be a config addon only (i.e. it would basically just be a folder with a file named config.cpp in it, wrapped up in .pbo form), but honestly it would be a rather superfluous creation. You can change the "side" of a unit/group placed in a mission simply by making sure the leader is of your desired side, even though said leader does not even show up in the mission. For instance, place down an East mechanized infantry group, and group it to a Colonel-ranked western unit with a probability of presence of 0%. Now the group will be on the West side, and fight against other eastern units. Simple as that.
Another option is to open up the mission.sqm and change individual units' sides from there., but then you can't 'open' them any longer, only move them around on the map.
If you still want to make the addon, you'll be looking at a lot of tedious copy-pasting, but that's about all you need to do. But, as I said, it's more or less unnecessary.
Wolfrug out.