Y cable is meant for splitting one signal into two, not the other way around (merging two into one). Although technically you can merge signals that way, it can cause issues which are beyond my understanding of electronics :) - loading, impedance, etc - google for it for more info. To merge two different signals (L+R) you need some kind of mixer. A simple passive mixer can be probably built inside XLR connector, to make this safe (again google for it).
I started a similar thread while back, on getting mono out of XLR. One simple way is to add FX loop in the last FX slot. Then use a short mono (TS) 1/4 patch cable from FX loop's out into FX loop left in. That will get L+R summed into mono and both XLR outs will be mono. Sure, you will use one extra FX slot, and you will also add one more D/A A/D conversion in the path, but it will work.
Other method is to add one of the mono FX in the last slot, something that will not colour the tone too much (compressor, or noise gate) which will make signal path mono inside POD. Again this take one extra FX slot.
zbf