I recall reading in an magazine a year or so ago, an article on this subject. One of the manufacturers (I think Boeing) has built additional fuel inlet points into the cargo area of 737s. They set up a framework with a collapsible tank in the cargo area and plug it into the fuel lines. This shows up as an extra tank. However, it might be that they have a separate indication of this tank and simply 'refuel' from it, rather than feeding from it. I cannot recall.
When the delivery is complete, the tank and it's framework are removed and sent back in a crate. The reason for adding the fuel access points is because short haul machines are now sold on more often in their life and so may need to change continents several times. This system allows any machine to be flown a substantial distance without modification and the extra costs of tech stops. Additional flight crew may be carried.
Airbus have extra fuel tanks that are the same size as cargo containers. So these get loaded into the belly, and then hooked up in the same way and filled. This is also used in the A319LR for permanent use.
I sit to be corrected.
