I think you just need to add a line like this:
PHP Code:
else if(StrEqual(strGameMode, "community5", false)) //Death's Door
gamemode = LMM_GAMEMODE_COOP;
In the l4d2_mission_manager.sp
Change "community5" for the tank run mutation name. (you can get the value by typing "!cvar mp_gamemode")
Let's suppose that the name of the mutation is "tankrun"
You would add 2 lines like this:
PHP Code:
else if(StrEqual(strGameMode, "tank_run", false)) //Tank Run for leaffan
gamemode = LMM_GAMEMODE_COOP;