fingal flyer
You asked about Fed Ex; here is the low down. Let's assume for the moment that the U.S. would accept the JAA license and would issue an FAA certificate; you still could not work for Fed Ex. All crews that are hired by Fed Ex are initially Memphis based crews and that would mean you would need to have a green card.
Take a look at part 61 of the FAA regs and you might find that you can take a written exam and pass a checkride in order to obtain a U.S. certificate.