The rules governing Ferry Piloting of aircraft (for example across the North Atlantic) vary slightly by the state of registry of the aircraft. However, where an additional fuel tank is carried (often within the Cabin), the installation requires a modification approval (CAA or FAA) and the use of the tank may well be subject to additional conditions.
This is a rigorously regulated field, and has little in common with the problem of wanting to carry a jerry can of fuel in your baggage compartment.