You can't do it with Afcad. You can easily do it with .xml code and Bglcomp. (for FS2004, don't know if it works in FSX)

You'll need to have the FS2004 Bglcomp.sdk installed, which you may already have if you use other programs to create scenery.

Copy the following text into notepad.
******************************

<FSData version="9.0" xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xsi:noNamespaceSchemaLocation="bglcomp.xsd" >
 <SceneryObject
   lat="N57 21.07"
   lon="E16 29.76"
   alt="0"
   pitch="0"
   bank="0"
   heading="94"
   altitudeIsAgl="TRUE"
   imageComplexity="SPARSE">

       <Trigger type="REFUEL_REPAIR" triggerHeight="0.0">
           <Fuel type="100" availability="PRIOR_REQUEST"/>
           <Vertex biasX = "-10.0" biasZ="-10.0"/>
           <Vertex biasX = "-10.0" biasZ="10.0"/>
           <Vertex biasX = "10.0"  biasZ="10.0"/>
           <Vertex biasX = "10.0"  biasZ="-10.0"/>
       </Trigger>
 </SceneryObject>
</FSData>

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Get the coordinates for the center of the fuel area from the sim and edit 'lat=' and 'lon=' using the coordinates.  Be careful that you enter the data correctly, no extra spaces or characters. Save the file with the '.xml' extension (example: XXXX_fuelpump.xml)

Drag and drop the .xml file onto Bglcomp.exe and a new .bgl file will be created (XXXX_fuelpump.bgl). Place the .bgl in the sim as you do other scenery .bgl files (in an active 'scenery' sub folder).

To see your new fuel area you can add/edit the following line in your fs9.cfg file.

[SCENERY]  <------to this section..
SHOWTRIGGER=1  <--------add/edit this line.

Transparent green walls will surround all fuel boxes. To remove them edit to SHOWTRIGGER=0

If you need to change the heading and/or size of the box, edit the 'heading=' and/or <Vertex........./> lines (defines the 4 corners-in meters from center). Again, be careful with your data.

This only creates the trigger area, it does not place a visual fuel pump. That you'll have to do with your object placer program.

Dave