Seems like you could modify a gas cap to allow pumping in fuel, assuming you don't want to make permanent changes to your tank and have a fuel bung welded on. Not sure where you'd put a tank high enough to gravity feed, but there are small 12 volt fuel pumps that would safely transfer the fuel. Kind of a question of how safe you want to be as to plumbing. Could use simple rubber fuel hose or hose with stainless mesh around it for a bit better protection. Hopefully your aux tank is metal of some sort. My main concern would be having some sort of 'breakaway' fuel connections that seal when they separate. In the event of a crash, and the aux fuel tank coming adrift, be nice if it wasn't splashing fuel everywhere.
In theory, could seal the main fuel cap, and vent it through the aux tank. So it'd pull fuel from the aux tank to the main tank automatically as it was used. But any slight air leak would interfere with proper operation.
I shudder when I see bikers with plastic cans bungeed to their seats or racks. In the event of a collision with a cager, I could see gas all over the place, not good.