How do I record a Mono Audio Signal?

Yes, that is possible, but it's a very complicated workaround. You have to open the recording from Character in Audition, after locating the file within the Finder or Windows directory. Files are named only after the time the take was recorded, so this can get complicated if you have many takes.  Once in Audition, copy the left channel to the right channel, then save it. Back in Character, you need to drag the edited audio file into a new track and align it with the original version of the clip you just edited. Then delete that original clip/track. Just note the waveform won't update, even after closing and reopening the session. It will, however, play on both left and right channels. Not ideal, and very time consuming, but it will work.

The new workaround I've discovered uses my Apollo Twin's I/O matrix. I can set Channels 1 and 2 within the IO Matrix to both look at Input 1 (where my mic is plugged in). If your audio interface has this type of control, this seems to be the best fix. I see no other way to change the Mono/Stereo state of audio channels within Character, either when recording or after the fact.