This should be dirt-simple:
Open the .scn you'd like to add tanks to. (You'll probably make a copy of a DG-IV scenario, renaming it).
Open the .scn with a DG and two drop tanks.
Copy the Tank1 and Tank2 vessel descriptors over to your DG-IV scenario.
If your DG-IV in the scenario is not called GL-01, change the name of the vessel the tank attaches itself to, from GL-01 to the appropriate name.
You're done. Should work, though I don't think I've tried that particular combination, and I can't guarantee that the DG-IV's more sophisticated systems management won't get confused or angry when somebody else goes adding tanks to it. Let me know how it goes!