You can jumpseat on any US airline ( with a reciprical agreement ) but you have to have a seat in back. You can ride in the cockpit only if you're CASS and the airline you are riding on is also CASS. The TSA allows international jumpseating in the cockpit on flights leaving the US, but not on flights coming back into the US. If there are seats in the back than you can still "jumpseat" on the airline.

I'm not sure on specific rules at Fedex on there jumpseat policies domestic or international.