Joint Stereo

Can a pre-existing audio recording have joint stereo applied after the fact?

I’m not much of an audio guy.



Yes, but why do you want that?
You can choose Joint Stereo as an Export option in Audacity 1.3 when Exporting as MP3 (using the recommended version of Lame for Audacity). However encoding (or re-encoding) to MP3 causes some loss in sound quality.