IIRC, this is already possible. The mapper only needs basic coding knowledge to do it.
However, he has to shoehorn his new roles into the existing slots. The only difference between his roles and the game's would be their name, loadout, and availability while the amount of roles he may have is probably limited by the game's current number.
Did not know that, thanks for the info.
Of course, it was so much simpler in the previous version of Red Orchestra.