Can't help you with the legalities or practicalities of carrying fuel in cans in the cockpit, but I can point you to something else.
Eventually you'll want to get the fuel in the tanks. Usually this means messing about with a funnel and sloshing fuel into it. Which may cause spillige and is also pretty dangerous since a static charge may build up.
A few years ago I was at the HISWA (boat show) and somebody had a real neat solution for this. It was essentially a 1/2" hose with a very crude one-way valve on one end, arranged so that the fuel could enter the hose from that side, but not exit.
You simply place the fuel container at a point which is higher than the fuel tank (in an aircraft that would be on the wing), dip the one-way valve end into the container, and put the other end in the aircrafts tank. Shake the one-way-valve up and down a few times so that the fuel is lifted up into the hose, and hey presto, instant syphon. His demo 5 liter container drained in 10 seconds or so.
No spilled fuel, no need to suck on a hose, no sloshing and the container is actually in contact with the fuselage, so not a lot of chance for a static buildup.
I still wish I had bought one of those hoses right there and then.