You will need to make the connection with a banded coupler. ABS and PVC being different chemically, can not be solvent welded together. I say this in spite of the fact that there is supposedly some kind of wonder product that will work on both. There should be no water, the trap is still below your connection point. Refilling with pea gravel probably is really necessary, but I might do it anyway. Often this space has a thin layer of concrete.