I'm doing this right now in my edit of Episode II. I split the 6-channel ac3 into six mono files, then imported all the tracks into premiere and put them on the timeline. I named the different tracks after what channel the file represented (LS, L, C, R, RS & LFE). If you then open the 5.1 Audio Monitor you can specify what channel the track belongs to, which'll make it sound right when editing, and then when you're done you can encode an ac3 from this, making the audio sound just right.