I would be very wary of such an endeavour. We had the same situation at our theatre and upon examination, I found that the wire used for the Express RFU was not suitable for DMX. Also, the lines were daisy chained to the various RFU ports around the building.
I'd suggest running new wire. Cat6 can be used for both DMX & for ETC Net3.
Thanks! After posting, I did more research, and found this to be the case. The cable running to the boxes is Belden 9728, and according to ETC specs, it should be suitable for a DMX output box. I’m just going to switch out the boxes and ends.