You can easily do this right in the MixConsole. On the Track you want to make mono change the Panner type from the default Stereo Balanced Panner to the Stereo Combined Panner. Now you can set both the left & right edges of the panorama to the center making your signal mono.
Going the other way, I do wish Steinberg would enable changing a mono track to a stereo track part way through the insert chain when required. Seems like a common requirement. It would avoid a lot of group/fx channels.
Or have I overlooked something which would make this a bad idea?
While looking for something else, I just found these Key Commands under Audio key commands:
Convert Tracks: Mono to Multi-channel
Convert Tracks: Multi-channel to Mono
So the can be run as one-offs or assigned to a Key for repeated use. This splits a stereo into two tracks or combines two mono tracks into a stereo track, or since it’s multi, I assume it can combine and split multi channel (5.1 or 7.1) but I didn’t test that.
Go to the audio connections and insert an unused mono output channel of your soundcard.
Now, go to your mixer and route the output of the desired stereo track to that mono output.
Render in place with the 2nd option and a new mono track will be created. After that, you have to change its output again to your stereo master out or group etc