You may have to bypass the grills regulator most likely. Usually right out of the tank, it goes thru the high pressure regulator lowering the pressure...now it doesn't have enough pressure to open the one on the grill. Look online, it's usually fairly simple to get a kit...plus you will need an extension hose anyway.
As far as back feeding, you could do it with the right adapters as long as you include a regulator between the portable tank and the connection. Swing by a propane place and they can probably get you what you need.