1) Use BeSweet to split the 5.1 AC3 to 6 mono WAVs.
2) Open any audio editor. Merge the Front right and Surround right into one right chanel, and the front left and surround left into one left chanel.
3)Now add the 0.1 track to both left and right tracks
4) I'm not sure what to do with the centre (or center for amerricans) chanel, but probably you can copy it to both left and right chanels.
5) Use the right and left chanels to do a stereo Wav file
6) Encode the stereo WAV to stereo AC3
That's a THEORY, I'm not sure how would it sound. Also you can in step one use besweet to convert the 5.1 AC3 to a stereo WAV (or AC3) but the results are really poor.
About Premiere - I don't know