Using Charlies method I would then delete one of the resulting tracks, as you only need one of them.
Another less obvious way to do this is to use the same 'Split stereo track to two MONO tracks' options BUT then in the resulting pop up, select the same track for the destination.
This 'merges' both of the stereo channels to one MONO track.

Image one below shows selecting the Track and the Split option path (via right-click)
Image 2 shows how to select the same track for both the L/R channel destinations (essentially a Merge)
Image 3 shows the resulting 'merged' MONO track ..
I used the 'split' function, but in the end result I was able to 'merge' the stereo track L/R signals to a single MONO track.
Hope that makes sense and helps ..