There are F/E's at Fed Ex and UPS who are not pilots, but they have been flying the line for many years as flight engineers. When they joined the airlines they didn't have to have pilot ratings, they just needed the Professional Flight Engineer FAA license, but that is no longer true of course.
Now all F/E's are S/O's, they have to take the F/E written exam to be considered for an interview but they need to have at least Commercial, IFR, Multi, and CFI ratings, and like in every airline there is a minimum number of flight hours a pilot applicant must have. You can't apply today at Fed Ex or UPS if you just want to fly as flight engineer all your life, all young flight engineers are pilots. They fly as S/O and two years after their date of hire they can move to the first officer's seat when there is a vacancy.
Two years ago when American was retirering the last 727's, there were still professional F/E's flying on those, they were over 60 and they had up to 40 years of experience in various aircraft types including the Electra, DC-6, B707, B747 and DC-10.
There was once in Airways an article about flying the IL
-86, which requires besides the two pilots a flight engineer and a navigator. The F/E had been flying in the engineer's station all his life in several russian models and the DC-10. If I remember well what I read in the article, that's the rule in Russia contrary to what it is today in the United States. When you start as F/E you do that all your life.
B747Skipper, happy contrails to you too.