Good point Mike! Additions have been added successfully for years, but this will definately alter the boats handling. Idle speed operation will not be effected, but cruising speeds will be... especially in varying water conditions. Planing speeds can result in excess lift at the transom, creating a broach.
If the new addition is "stepped", or recessed befind the original transom, you can offset some of the unwanted handling characteristics, however this area will continue to run wet and therefore it will have an overall effect on handling. There have been numerous cockpits added to Hatteras's over the years. As Mike suggested, choose a reputable yard with *specific* experience in performing this modification.
If you can wait just a little longer, we have an advertisement coming out in Hatteras World magazine shortly, promoting YachtForums to Hatteras owners. The issue will reach close to 20,000 owners, dealers and prospective customers. We're expecting a little Hatteras Owner traffic from the same. Hopefully, one or more of them will have experience with the addition and can provide some solid advice.
We had a 57' Hatteras Extended Cockpit a few years ago. The boat was a dream to handle around docks. Sure miss the aft cockpit!