You can do all this yourself without a
school! Apply directly to the FAA online, to have the
FAA verify your EASA license with
this form.
Once your EASA ATPL has been verified by your issuing country, the FAA will contact you by email, then you need to make an appointment to a FSDO (in the US city/state of your choice).
During the FSDO in person meeting, and after reviewing all your [original] credentials, the FAA Inspector will tell you exactly what you need to do.
E.g,: you may, or may not, have to take the FAA ATP written exam, if you do, you can easily study by yourself with an online course (see suggestions below). You also might (or not) have to take a CPT course from an approved provider (
Certification Training Program - there are more providers that subcontract from the approved ones in the list). You probably won't need a TSA authorization because you are already a licensed pilot (unless you want to do some
flight training in a
large aircraft or a
turbojet).
Again, avoid so called
schools and specially those for
foreign students! Unless you have money to waste, and want to do all that is not required (student visa, TSA, expensive ground school, lots of flying with an instructor, etc.). That's how these schools make their money, and they know how to milk foreigners who don't know what they don't know.
This said, there are many serious training providers in the US. One is
ATP Inc. but I would not use it in your case, because you said you already have an EASA ATPL.
If needed, these are IMO the two best self study ATP written exam prep courses:
https://www.sheppardair.com/atp.htm and
https://kingschools.com/Search.asp?code=atp