Do you have each channel on its own track in Premiere? You should be able to mute all but one, export a mono WAV or AIFF of that one channel, then repeat for the other five channels. You can then load up your individual mono channels in one of many free programs to render a 5.1 AC3 track.
I don’t have each channel on it’s own track, is there a way I can separate the channels in premiere?