What is the procedure to obtain an FAA ATP while in the United States for a holder of a CPL from Pakistan?
There are many things that need to be accomplished.


Get a letter of authenticity from the FAA validating your license is valid and from an ICAO contracting country.
Study and pass the ATP multi-engine written test.  You may have to get a graduation certificate from a school offering the CTP course.
Apply for and receive TSA clearance.
https://www.flightschoolcandidates.gov/afsp2/?acct_type=c&amp;section=WN
Take the FAA ATP practical test in an A320 airplane or FFS simulator.  It is best to contact a training center on what is required.