FedEx has both line pilots and professionals working as simulator instructors. I just talked to a friend of mine today who's a pro, and the application process for professional instructors is to apply for a vacancy through the FedEx job finder (google it for the link).
As far as moving from professional instructor to line pilot, it depends upon the contract they hire you under -- some instructors have been guaranteed line jobs after three years, some have been hired as permanent instructors only, and everything in between.
Finally, disregard inline's comment about needing a military background -- if you search his post history you'll see he has some problem with FedEx's policy regarding hiring military veterans. All of the professional instructors I've dealt with have had pure civilian backgrounds.