I think you'll find you require two checkrides - one FAA and one JAR. The last time I added a type to both licences (March 2007) I did the FAA ride as a preparation for the JAA ride - the day before. FSI is very good with this - but it may not be the same examiner. Just let then know what you want in advance and they will make it happen.
As I recall the FCC fee was $50.00 - you can do it all online.
Good luck!