You certainly can run a ground circuit to this light w/o having to change out the wiring. Simply pull another wire and use only the ground. However, this never proves to be an easy task. As Glennjanie's mentioned; don't worry about the ground, just install your new fixture and ignore the ground wire on your new fixture. Chances are that the new fixture won't be wired the way your old one is. Your only going to need one of the black and one of the white from your old wires. Basically, remove the "Y" from the old wiring and use just the white & black coming out of the wall. If these leads don't have enough slack in them, splice on enough to make your connections and eliminate the other leg of the "Y".