If the Croation CAA are a full JAA state then you apply to them on a case by case basis to be approved for conducting tests. I am a UK FE and am approved by a number of other countries, including Spain, Portugal, Italy, Czech, Poland, Germany to name but a few! I spend a lot of time doing skill tests in other countries.
If you are talking about being issued with an examiner authority directly by them then you have to apply and meet whatever standards they set which will generally include being based in that country with appropriate business connections.