Well, that depends on what you're talking about.

Do you mean:

Add a fuel tank to the exterior model of the aircraft?  If so, that isn't possible without the source files the aircraft was created from.  You could try contacting the author and asking about the change.

Adding an interior (not visible) fuel tank?  That you can do just by adding a few lines in the aircraft.cfg file.  Look for an entry labled [fuel].  You can either change the amount of fuel in the existing tank(s) to be what you want, or you can add a tank just by copying the last line and increasing the label number.  Then change the amount of fuel in the tank to be what you want.  I believe the last number in the line is the amount of fuel.

Baron,
The last figure in the line is the number of gallons of unusable fuel - the number before it is the usable fuel figure. The three numbers before that are the horizontal, lateral and vertical locations of the fuel tank. Adding an extra tank might affect the plane's balance, so you might need to compensate for it.

An alternative is to adjust the fuel_flow_scalar figure in [GeneralEngineData]. It's defaulted at 1, but the more you decrease it, the more efficient the engine becomes: reducing it to 0.5 makes the plane fly twice as far, for example.