You won’t add, convert, or surrender your FAA Certificate. You obtain a separate airman certificate. Countries around the world have their own unique requirements. As a summary, you will need to:
Present your log books for review. Be prepared to be without your log book for upwards of two weeks, depending where you are seeking a foreign certificate.
Submit to medical examination which is extensive.
Take as many as 14 written exams for the ATPL. Some countries may waive some exams depending on your experience.
You will have to take License Practical Check (LPC). CAE, and Boeing Alteon can provide assistance with the preparation, and practical exam.
Again the above is an outline of requirements. I recommend you look at some websites for more accurate details. They also have phone numbers so you can talk directly their licensing departments.
UK CAA
Civil Aviation Authority Home Page
Irish Aviation Authority
Irish Aviation Authority
CANSO Civil Air navigation Service Organization
Canso - Civil Aviation Administrations (CAAs)
An alternative is to apply for a validation certificate, usually good for 6 months in duration. The requirements are not as arduous. You still need to pass the medical. Usually two exams are required… Air Ops, and Air Law. The LPC is still required. Again you can contact the licensing department of the country’s agency directly for accurate information.
Hope this helps. Good luck and happy flying!