ABSOLUTELY POSITIVELY NOT!!!
seriously dude. Your water heater is exhausting carbon monoxide and the flumes on water heaters and high efficiency furnaces are very sensitive to different back pressures. If you tap into that exhaust, (even if you tap in with a Y and not a T) your going to be pushing deadly gases back in to your living space.
Surely you can find another vent. Does it vent into a chimney that you can run another line into?