It's possible in Vegas, which can create 5.1 soundtracks. My receiver does not have a Prologic II, so I took the stereo soundtrack, ran it through a software Prologic emulator and create 5 audio streams (C, L, R, BL, BR). I dropped these into Vegas with the original video/audio, synced them up, and created a 5.1 soundtrack. I received quite a few compliments on it.

I don't think this would work with the straight mono soundtrack (which wouldn't have the PL steering instructions), so each channel would have to be recreated (or layed over the earlier PL soundtrack). It would probably take awhile, and probably not worth in the effort, IMHO (Too much chance of becoming a mish-mash.)