All 121 pilots are required to hold an ATP with a type rating under the FAA system. The ATP is just a COMM with an IR held to a higher standard. If you hold a foreign ATPL you would be eligible for a FAA ATP. You would have to have your CAA verify your ATPL to the FAA, complete a weeklong ATP Certificate Training Program (~$5000), pass the written test and pass a checkride. Many of the regional airlines provide the CTP as part of their new hire training for pilots who only hold a COMM+IR. Don't know if they would provide it for a new hire transitioning to a FAA certificate.