If you assume that the round-trip fare between AAA-BBB is the same as the round-trip fare between BBB-AAA (and most of the time, it is), then you can just do your dream maps from GRB and all of the fares displayed should also be valid (with very few, if any, exceptions) to GRB.

I've used this method several times with trying to match up two fares into one (i.e. ATL-XXX, XXX-LAX). First I use the ATL dream map to see what's cheap from ATL, then the LAX dream map to see what's cheap to LAX. If I find the same city in both lists, I'm golden.

And welcome to FlyerTalk!