Use one of your MIX busses setup as an AUX (all POST fader). Route that aux to one of the local output sockets, then connect that socket to the input of your external reverb unit.
Next take the L/R out from your reverb unit, and run those into a pair of local inputs which can be mapped to a stereo channel (or 2 mono channels).
Go to the mix that sends to your external reverb, and raise the levels on channels that you want reverb on. *Make sure that you don’t send the reverb return channels back into this mix or you will end up with a feedback loop.
Finally go back to the LR main mix and raise the channels that have the return from the reverb, and blend to taste. 😀