Sonar 5 producer. I inadvertently recorded an audio track with the input setting on 'stereo', resulting in the data only being recorded on one (left) side. (I was only using one line in to the interface) Is there any way I can copy this data onto another (blank) track in such a way that it can exist on both left and right sides? Or do anything to rectify this situation other than re-record it with the proper input setting?

Bounce the track making sure to set the channel format to "Split Mono" in the "Bounce to Track(s)" dialog. That will give you two mono tracks which is what you really wanted in the first place. Throw away the empty track.