So how do they get aboard? In a cargo container? And once they kill the crew, how do they get clearance to take off? I can just imagine the conversation with the tower:
FED-EX 123: "Ziss Ez Fez-Ex wun too zhree. I beg your ezcellenzy for clearance to taxi to runway zhero-zhree-zhero for launching."
TOWER: "Billy-Bob Baker, whassup? Stop with that fake accent, man."
FED-EX 123: "Zorry. Weel repeat. Need your ezcellenzy to allow clearance to taxi to runway zhero-zhree-zhero for launching."
TOWER: "Ah." Long pause. "Copy that Fed-Ex 123. We have just had an inbound jet declare an emergency. Go to Taxi four-five-four, and hold. Repeat, go to Taxi four-five-four, and hold, until the inbound has landed."
Sounds of fire trucks rolling, then blocking runways. Crash wagons roll, and park themselves behind and in front of landing gear on Fed-Ex 123.
To be honest, I find this a lot less far-fetched than the idea of a bunch of Arab Ninjas secretly sneaking aboard an airplane, killing the crew, and then routinely taking off.
The funny part is that any pilot knows those stupid idiots cannot even do a readback without assistance from the controllers. They freak at the most basic radio request for expressing an intention.