Fuel is a lot denser than cargo tends to be; I suspect this would be harder to do safely than you might expect. You'd need to make sure the balance is okay and stays okay throughout the flight (can't have it slosh around).

You could surely get Boeing to retrofit one of their planes like that, but I don't think it is something a 3rd party could slap together on short notice.

Probably not. Most airplanes are at maximum takeoff weight when the fuel tanks are full and they are loaded with passengers and cargo.