Unreal Kingdom has a reply from Tim Sweeney to their question about running single player Unreal maps on Unreal Tournament (thanks PlanetUnreal).

UT will ship with all the Unreal 1 sounds, textures, and models, so the maps *might* work. We haven't been testing the Unreal 1 game code in the UT codebase for months now, so I can't promise that certain aspects of the gameplay haven't broken, but at least everything compiles and seems to work at a glance. So I expect a lot of the U1 single player user maps will work with UT right away, and if some break, they can probably be modified to work.