I did it about two years ago and managed to put my Hong Kong 747 rating onto my JAA ATPL. All it involved at the time was renewing my JAA Class one Medical and an initial 528/IR style simulator that took around three hours. Rented the sim from Virgin and actually had the Chief Pilot of Virgin 747 fleet doing the check who was absolutely brilliant. I remember all I needed was a TIRE to do the check and not have a Campaign Against Aviation (CAA) examiner present in the sim as well was due to me already having a current (Within 5 years of last renewal) on Multi Engine piston, but It could have been a Single Engine piston or 737, A320 etc. As long as it was within five years. Pay the money to the Counter Aviation Authority (CAA) some money submit the forms and you will have it in a few weeks. All in all an expensive experience. Don't quote me as this was done as I said a few years ago and things might have changed under EASA.
The Cav