This is something I've been curious about as well. As I've seen mods for Zoo Tycoon that port from Civilization(completely different publisher and dev), I've seen porting from Oblivion to Jedi Knight(and those even got uploaded to k-files and no one cared.)
I've taken part in a few different modding communities, but this is the only one( where I've been told NO PORTING, which always made no sense to me, as whatever clause is in the EULA saying don't do it is invalidated by us as soon as we start modding anyway.
Plus is EA/LA really gonna see a new mod and go "OH DEAR GOD, they used a K2 droid in K1, sue them all!"