Hi Ifresh,
ADDING TANKS TO THE VISUAL MODELI'm afraid the answer is "no". It's impossible to make any changes to the visual model unless you possess the original 3D model (i.e. the Gmax or 3DStudio source files created by the author).
The 3D models are never released to the public, and there is no conversion program I am aware of at time of writing that can convert .MDL files back into editable 3D models.
Also, I wish there was but as far as I am aware there is no MDL editor for FSX available that would allow you make small tweaks directly to a model (for example, that might allow you to add or remove attach points in the model for effects etc).
The Xload F18 is based on the Acceleration Pack F18 model but has been developed much further (Captain Sim made both models). It has virtually exactly the same FPS performance in FSX.
ADDING FUELOn your question about adding fuel in the aircraft.cfg file; there's quite a lot you can do. All the editing is done in the [fuel] section of the file.
There are two threads in this forum about it. They give usable examples for adding fuel to the physics model, including the coordinates of the tanks. (the coordinates affect the centre-of-gravity of the aircraft, they are not related to the visual model)
http://www.fsdreamteam.com/forum/index.php?topic=359.msg2075#msg2075and
http://www.fsdreamteam.com/forum/index.php?topic=424.0The multiplayer model appearance is discussed in one of those threads too. For those who don't have the model it appears as a Learjet 45.
Hope this answers most of your questions.
Regards,
Voodoo