Assuming the airplane use the same internal variables as the standard version, just with a different doors config, the easiest way to copy the configuration for an internally supported airplane to a new variant that stays in a different Airplanes folder, is as follows:
- load the supported version ( the standard NGX in this case )
- open the GSX airplane configuration editor and make a *very* small change, one that won't be visible, like moving a door one centimeter in one direction, so it will trigger the "Save" button to be active. You can even restore the previous value as if was, as long you can active the Save, it's fine.
- Go into this folder:
%APPDATA%\Virtuali\Airplanes
You should see a folder named like the version you edited, for example "PMDG 737-800NGX". Using Windows Explorer, Copy and Paste back in the same place the whole folder, so it will create something like "PMDG 737-800NGX (copy)"
- RENAME the copy exactly like the name of the folder under Simobjects\Airplanes of the new variant.
- Go back to the sim, and open the new variant.
- Open the GSX airplane configuration editor again, you should see the airplane is now configured using your new config, and make the necessary changes, like removing unused doors.