All you need is notepad.
Open the aircraft.cfg file located in that aircraft's main folder.
Scroll down until you see [FUEL].
There are several tanks in there. The following is from the stock FS9 737:
Quote:[fuel]
Center1 = 0, 0, 0, 2313, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
LeftMain = -3, -19, 0, 1500, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightMain = -3, 19, 0, 1500, 0 //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
fuel_type = 2 //Fuel type: 1 = Avgas, 2 = JetA
number_of_tank_selectors = 1
electric_pump=0
If you want to add another center tank, add and entry that resembles the ones above, but the first part would be "Center2". Then take a look at the stuff after the "//" sections. That will tell you where the tank is located in the aircraft. The numbers correspond to the listings after the "//".