When you export stems from Studio One the number of channels for each mixer channel are already set by the instrument. If the instrument channel in the mixer is mono (most multi-output drum samplers have mono and stereo outputs where the Kick and Snare may be mono channels but the Overhead and Room channels are stereo) it will render those mono channels as mono audio files if the option "Preserve Mono Tracks" is on in the dialog.
If you can configure your virtual instrument (internally) to only ever present mono output into the Studio One console it will always only render mono channels.
The other options are to use a mono bus (switch the group bus to mono and send it there) and render the summed mono bus channel.
Of course you can always bounce any two channel audio clip(s) to true mono after the fact in Studio One by switching the track mode to mono, trimming the clip gain by -6 to account for the increase in gain when summing to mono, and hitting Control + B on the clip(s).