Yes, you should be able to modify the unit so that one of the 1/4 outputs, either on the front or back panel, becomes a summed mono output while the other pair is still stereo.
In fact, I don't know if the Stereo/Mono switch makes ALL the outputs mono or only the front one...?
Usually the request is the reverse, FOH isn't interested in stereo but the player wants to stand in front of it...
Jimmy J