Generally speaking, there's two things to be said about this:

• Obtaining an EASA certificate in addition to your FAA one is a smart career move. It gives you a lot of flexibility, employers (including ones in North America) love it, the process will give you operational knowledge that you didn't otherwise, and in general it will give you a real edge over those who don't have it.
• Converting to an EASA license is not particularly easy.

There's also one very big certainty:

• Almost without exception, you will need to take and pass the 14 EASA ATPL theory exams if you want to convert to EASA ATPL (Airplane).

This is where our apps come in - we help you pass those 14 tests.

However, your question probably deals more generally with the process of conversion. For this, we must refer you to one of the schools or training organizations that actually handles conversion. It would be improper of us to name one here, but there are several online. Basically, the process varies significantly based on the existing licenses and certificates you hold. Furthermore, the process tends to change from year to year (sometimes slightly, sometimes a bit more), and so we really do suggest that you get the best info right from the people who deal with the specifics every day.

(But don't forget to come back here for our EASA ATPL exam prep apps to help give your study a real boost!)