A game creation package like FPSC is certainly possible on the 360, like if TGC made a version for the 360, they could have Live enabled deathmatches and stuff like that. That would be great, it'd sell like hot cakes as well, but it unlikely to happen unfortunately.
But there's no real way to convert your FPSC game to work on the 360, and probably never will be. Someone would have to literally re-code the whole engine for the 360, it could use the same media and maps, but the actual engine executable is nothing like what the 360 uses.
We're going down... in a spiral to the ground...