I have, and it's fine. A lot more work involved as there isn't a lot of examples like there is in UT3. So unless you know the structure (and judging by your past threads, you guys don't) of Unreal Engine well, it will be a hard game to modify.
Yes, you have to buy RobobBlitz to have access to Unreal Editor within it.
It's a cheap game, and if the modification is good enough then people won't mind buying it to get the modification.