From the manufacturers website: "We want to help you adapt! Mayfair Games likes to offer our customers all the latest possibilities in product compatibility. That being said, we are excited to announce that we will be offering customers, who still own the Catan 3rd Edition games, The Settlers of Catan Pre-2007 Adapter Kit. The 4th edition of The Settlers of Catan (Settlers-2007) provides an outer frame that encloses the 19 hexes that make up the island of Catan. This adapter kit provides that frame, the frame extension pieces for the 5-6 player extension, and the harbor pieces that allow you to vary the harbor locations from game to game.
This adapter kit also allows you to use the Catan: Seafarers 4th edition with 3rd or pre-2007 editions of Settlers.