The BBJ may look like a 737-700 with winglets, but underneath, it's an entirely different bird.
It has the wing of the 737-800 to carry the extra weight. It has extra auxilary fuel tanks in the cargo hold. Of which the customer can choose the number of tanks. After the customer takes delivery of the aircraft, the customer is responsible for the interior completion of the BBJ. Versus an airline, when they take delivery of a standard airliner, the passenger interior is already completed.
I'm not a real aeronautical engineer, I just play one on Airliners.net.