Welcome Glaikt,
I think the best way to achieve this is by using MIR Pro as some kind of "glorified AUX-send reverb". :-) I'll give you the step-by-step instructions, but they make things looking more complicated than they actually are, so don't despair:
1. Instantiate MIR Pro in an FX channel, much like you would do in case of a conventional reverb plug-in.
2. Select the MIR Venue of your choice, with a microphone setup that fits your needs (most likely Stereo M/S, maybe a Blumlein-array of crossed Fig-8).
3. Move the MIR Icon to the center-axis of the stage. (... you can do this by typing-in the coordinates, too!)
4. Select a General Purpose Instrument Profile, with Omni or Wide Cardioid characteristics.
5. Increase the stereo width of the MIR Icon until the left/right handles form an equilateral triangle with the Main Mic's position (... you don't have to be too finicky about that, it's just a rule-of-thumb).
6. IMPORTANT: Set MIR Pro to "Wet Solo" in its Output section!
7. ... now route your mix to this FX channel and add the resulting reverb according your to taste and needs. The original stereo width of your mix will stay unchanged.
HINT: Keep in mind that changing the Icon's stereo width and/or its position on MIR's stage will give you almost countless options to sculpt your sound. Don't forget that you can shorten the overall revereb length! And finally, there's always the RoomEQ to get rid of boom, disturbing resonances and so on.
HTH,