Can a pilot work flight engineer's station? In kinda reverse of Did Flight Engineers progress to be pilots?

In three-crew cockpit can a person qualified as pilot work in the position of flight engineer? Does a flight enginner have different qualifications that are not covered by an ATPL, thus rendering a pilot unqualified for the position?
The pilot cannot just sit down in the flight engineer station.  First there is a different FAA flight engineer certificate that the pilot needs to obtain.  Secondly, the pilot would need to go through the airline's training program for that station.

Granted, a pilot who has flown that airplane before should quickly get those done.