Wiedenhopf
The B737NG centre tank is not inside the pressure vessel, it is infact just like the wing tanks in that it is in the area between the front and rear wing spars and reaches across the Aircraft from just inboard each engine pylon.
The extra fuel carried in the BBJ is in tanks contained in the cargo holds that are pressured. The fuel from these tanks is transferred into the centre tank by regulated cabin pressure so that it can be used in the same way as it is in the Airliner variants.
The fact that the tanks in the cargo holds are at ( regulated ) cabin pressure and the Center tank is at ( slightly ) above ambient pressure allows the fuel to be transferred into the centre tank without having to resort to fitting transfer pumps or boost pumps and modifications that would vastly complicate the fuel system and take it away from the airliner standard.