Yes and no. Comms are often run in 4 wire mode on large events. As I mentioned earlier I have an annual event where I run 14 channels of comms through a Dante network, but there is more involved than another power supply. For each channel of 2 wire comms, ClearCom, RTS, or any compatible, you need a send audio path and a return path. You also need, at each end, a 2 wire to 4 wire converter to change the 4 wire balanced audio send and return pair back into a 2 wire common talk/listen bus unbalanced system like ClearCom or RTS. On the 2 wire side of each of those 2 converters you need a local power supply to power the belt packs. Where you now have a single power supply that powers all your packs, and a single XLR to run to the stage, you will need 2 power supplies, 2 4 wire converters, and 2 channels in your digital snake, one in each direction.
In the case of the show I do it on it would be very troublesome to connect everything via the unbalanced powered comm lines. We are pulling power from all over the building and we have no copper infrastructure between floors. We have plenty of fiber, and a Dante network that goes everywhere, and we are interfacing with a Riedel comms matrix that is already a 4 wire system.. It is much better for us to put the 4 wire converters at the far end and run balanced line level through the network to the Riedel than to run analog cable and interface in the control room.
In your case you are talking about a single XLR, less than 100m long that runs along with the CAT5 for your digital snake. Before I went out and spent many hundreds of dollars on the gear to eliminate one XLR cable, I'd tape an XLR to your CAT5 and run the bundle as a unit. I'd probably also tape in at least 1 additional CAT5 and maybe another XLR as back up.
Mac
Thanks to everyone who replied-
My head is swimming....
Since this is just a one-to-one situation (FOH to behind stage), I think the cheapest solution is the XLR cable, since I already have that.