Mark Charters posted a message about Re: Adding Sides to Red Alert 2 Yuri's Revenge in reply to Re: Adding Sides to Red Alert 2 Yuri's Revenge
Unless you hack into the EXE or I think change the Observer side into a new side its pretty much impossible. And it will be very buggy and you'll need to add new AI etc too. Also you can't convert the Mutant / JP / Civilian sides etc to your side as they don't use background images like the Victory images for a Skirmish. Thus the game will crash.