Bahama,
Do you currently hold a FAA certificate? If you do, you will need to do a conversation. And there is a paperwork shuffle you will need to do. Go to the FAA website (
FAA: Home) and follow the process. Since you have instrument privileges, you will need to take an instrument knowledge exam. I believe there is a specific exam for this, rather than the normal instrument knowledge exam. Then you will then visit a FAA Flight Standards District Office (FSDO) where you will present your documentation and be issued a private pilot certificate with an instrument rating. It will be tied to your national certificate. You will then do you ATP practical test and be issued an FAA ATP without restrictions.
Keep in mind that the ATP standards for the Airplane ATP will change in July 2014. Read FAR 61.155, 61.156, 61.157, and 61.159. All your SIC time does count but you will need some PIC time. If you have done Sim training in the Dash 8 make sure to is properly documented. Much if not all of it should count to meet the new requirements.