Possible; safety is key.
I would go to your local LP vendor. He should have all the correct parts. You don't need to tell him exactly what you are doing. He may be cool with it, he may not. We do it all the time. The one thing you have to do while filling the tank is vent it. There is a little straight blade slotted screw on the side of the valve. Open this screw slowly while starting to fill the tank. Open it too far and you'll spit LP liquid. You just want it open a little. When the tank is full you'll get liquid out the screw; then tighten it and shut down the fill valve. Nuthin to it. Just keep the 20lb cylinder lower than the top of the bulk tank. I would add an additional fitting onto your bulk tank that stays there. Just cap it when not being used. The fitting you'll need for the 20lb will be left hand thread flare. It will thread into your 20lb tank fitting opening.
Obviously, no smoking, no open flames, yada yada.
NAR 73787 L2
Tripoli 13524 L2
QCRS, Princeton, IL
Buckle up back there, we're going into "Hyperactive!"