If you're storing it in the tub, fill from the top and pump from the top, that way it can't feed into the tank until you're ready.
If you want it to simply add capacity to your current tank, the bottom of the keg will have to sit slightly above the bottom of the current tank so that the fuel pickup is where the last of the fuel sits. A hose at the very bottom of the keg will feed fuel out to the main tank, and the filler moves from the main tank to the keg, so that you fill it first (and it fills the main tank). An additional breather is required from the main tank to the keg, and the main vent (that plumbs back to the top of the filler neck) needs to go into the top of the keg (not the main tank - you need to block that off).
Aux tanks aren't easy. Mounting it will be fun, too. You're hanging what, about 10kg of tank and 46.75kg of diesel? Call it 60Kg that you have to support over any terrain without movement and not too close to the exhaust.
Sounds like a great idea, I hope you get it to work!