You wish dude. You charge directly for anything related to the use of relic copyright, and you can be taken to court. There was a guy who made a map pack for duke nukem. All he did was put a lot of privately created duke nukem maps and sell em as a pack. it was deemed an infringement since you needed the copyrighted duke nukem game to use em afaik. I've researched mod copyright law , it has a few cases from the american system. im looking for the stuff now.
If you use a map editor you use content from the game. It's not the same as creating content for a game.
It would be more as a perk for donating. If relic didn't wanna lose out on money they wouldn't allow us to have donation in the first place. Paying money to use other peoples content is already what donators do.
Also about that duke nukem lawsuit. The creators were selling the maps as a stand alone game on PC CD-ROMS in stores so you never had to buy duke nukem to play it cause it was included on the CD.