Pol01,
you might be better of asking this question in the Engineers & Technicians section of Pprune.
Unfortunately i can't tell you the requirements to become a flight Engineer. However once upon a time i played with the same idea and after doing some research decided against it. Biggest reason for voting against it is that there are not an awfull lot of flight engineers around anymore and their number is decreasing. If you choose to go ahead with it than be prepared that the chances of finding a job in europe are very small. I think you can forget about the States also (greencard etc.) So you are looking at a job in Africa or the Far East.
I havent answered your question but hopefully it helps a little bit.
Flying Torquewrench