What are the requirements to convert an FAA certificate to EASA/JAA?
As @Pondlife states, this will depend on each member state's implementation of the EASA rules but I would expect that the implementations would be similar -- otherwise what is the point of EASA?  (That would be a good question except it would no doubt be closed as being too "opinion based".)

I've added a link to the relevant UK documentation and marked this as a Community Wiki answer so that hopefully others can expand on this for other states and perhaps add personal experiences.

UK

CAP 804 Section 4, Part Q deals with "Validation and Conversion of Non-EASA licences issued by States other than the UK".