Can I take the FAA CFI knowledge exam(s) based on my EASA ATPL knowledge?
There is no official theory exam preparation process that is required to be able to take the test, you can do all preparation on your own. If you are asking if having passed the EASA ATP theory exam means you don't have to take the FAA CFI then the answer is no, you definitely would have to take the exam, although I don't think that's your question.

I think what you're asking is if the material in the EASA ATP is the same in the FAA CFI and you don't need to study. While the material is very similar and there will be a lot of overlap the two are not the same, there will be questions which ask specifically about FAA regulations, testing standards and student training which are not obvious and you are not likely to pass without knowing the material.