I was wondering if it was possible to basically take just one of the channels from a stereo mp3 and turn it into a mono mp3 without transcoding?

The mp3 in question is encoded in regular stereo, there is very little difference between the two channels, except they seem to be 180 degrees out of phase.

Technically yes it should be possible, but practically no since there is no software that are capable of that. At least to my knowledge.