That's not quite how it works. In zero g, just adding Helium pressure to a tank won't accomplish much. You either have to use some kind of pressurized bladder to force the liquid down (ok for thrusters, too big a weight penalty for the main engine fuel and oxidizer) or supply a small acceleration, say from auxiliary thrusters, to settle the liquid to the bottom of the tank prior to ignition. Then He pressure can push the liquid into the main pumps which, in turn, provide enough pressure to force the liquid into the engine against its internal pressure.
Ullage motors are used to force the propellants to the bottom of the tank...mostly