It's rather simple. I do the same thing Gary does. e.g. you want to launch one of your DGIV's on the Hyperion launch vehicle. Seperate .scn file has the general setup in it, you just change the name of your DGIV to the one in your main scenario file.
Easy as pie once you get the hang of it.
I do the same thing with crew members with UMMU.
Just make a list of the people you use or even group them as in to separate teams.
For this example let "OnOrbit" Scenario be your current save (the one you use to save over multiple times). "Teams" would be for my crewmember setups.
You would copy and paste which ever team you wanted to use into your DGIV scenario code. Works the same with rockets. All just copy and pasting. Think of it as "Analog KSP"
Some add-ons actually offer an in-game payload setup. Such as Kulch's Energia rocket, and more recently, Loru's/Woo482's Hyperion launch vehicle. If you want something KSP like for easy visual rocket setup, use that.
All in all, everything you do in Orbiter can be done in the scenario editor. It's just a matter of knowing where to write certain code lines etc.
Take a look at some scenario files and note how everything works. Notice attachment points and docking points. Once you figure out how those work, you're pretty much good to go.
Jamie