If you want a low-pressure line for the grill, you could disconnect the furnace and water heater from the existing adapter tee fitting, remove that tee fitting, add a black iron tee and a couple of short nipples, and put the existing adapter tee back on one of the nipples and connect an appropriate flare adapter fitting to the other nipple for the grill.
You'd need to bypass the existing regulator on the grill. On some (mostly inexpensive) grills that's not readily doable as the regulator and control valve are the same unit.
