Delving into JAR a bit more it appears that they only cater for licences issued by non-JAA States to be rendered valid. If you have an ICAO compliant National Licence you have to convert it to a JAA one.
JAR-FCL 1005(b)
(3) Holders of a licence issued in accordance with the national regulations of a JAA Member State before 1 July 1999 or in accordance with (b)(1) above, may apply to the State of licence issue for the issue of the equivalent licence specified in JAR–FCL 1 (Aeroplane) which extends the privileges to other States as set out in JAR–FCL 1.015(a)(1). For the issue of such licences, the holder shall meet the requirements set out in Appendix 1 to JAR–FCL 1.005.
Haven't looked but imagine it's the same for helos.
Mike