Back to the original question, if you can solder you can build a Y cable with resistors that will sum left & right to mono without messing up the stereo signal. By including the resistors you are essentially building a fixed-value 2 to 1 passive mixer.
For the left and right pre-out RCAs, solder a 2000 ohm resistor between the center pin and the (+) lead of the cable. For the mono input RCA connector, connect both left and right (+) leads to the center pin. All ground leads are connected to the RCA barrels as usual.
For a balanced to balanced XLR summing cable, use a 1000 ohm resistor on pin 2 (+) and pin 3 (-) of the L & R pre-out XLRs and connect the ground lead directly to pin 1 (no resistor). For the mono input XLR, solder both (+) leads to pin 2, (-) leads to pin 3 and ground leads to pin 1.
You can build single ended cable with XLRs on one end and RCAs on the other. For the XLR use pin 2 for (+) and pin 3 for ground. Use the same methodology as above soldering 2000 ohm resistors to the pre-out connector (+) pin and summing at the input connector (+) pin. The XLR pin 3 ground lead may or may not need to be connected to pin 1. I usually solder a bridge wire between pin 3 and pin 1 and plug everything in. If the rig is quiet, then I leave it. If I get a ground loop hum, I open the connector and snip off the pin 3 to pin 1 wire.
Gear with XLR connectors should have specific wiring and ground instructions in the manual, so check that first before you do anything.
Russ