i was told by the officer in the Miami FSDO office that you cannot add additional FAA ratings, i.e. Instrument etc.. onto a FAA licence issued on the basis of a foreign ICAO licence, ie. i could not do a FAA instrument rating on my FAA Private that was issed against my JAA PPL.
He said i would need a full FAA Private (i.e. take the Private written, oral and flight test)