Actually, it's pretty simple to convert an FSDS source file to dxf so that you can import it into gmax.
Just download the CVA stand alone converter, input the file location of the fsds source file (*.fsc), set the name of the new file (suffix dxf.) and "convert"
Of course, if it's
this one that you're looking at, remember it's an old... OLD model (originally made with either Flight Shop or Aircraft Factory 99), with animated nothing ...
You can still use it as a "backdrop" of sorts, against which to build your new model.