When you use the template of your mixer in a song, all inputs are routed to their appropriate tracks in Studio One - and back! This means that the SL 32 is receiving the processed signals and can send them to the 16 flex mixes and the 4 fixed subgroups. You can also create busses in Studio One and route them to inputs of you mixers. As far as I know and see a direct routing of busses to flex mix outputs is not possible.
You may want to ask specialists, which you can find in the closed Facebook group "PreSonus StudioLive Series III DAW Mode", https://www.facebook.com/groups/SeriesIIIDawMode