Normally sound in UT3 is rendered into 5.1 from mono and stereo samples in the environment and soundtrack.
Unless you're trying to import 5.1 sound for the music of a level, you can probably recreate the effect by importing pairs of sounds as stereo and using 3 sound playing locations to get the full effect.... but the positional audio system should be able to render however many channels from stereo or mono sounds, so it wouldn't be really necessary.
If you are trying to import 5.1 channel music, I would say use your music program to condense it to stereo, and if you really want to try to add a sub channel, you could try a huge radius looping ambient sound or something. It would be difficult to get the rear and center channels though.