If you open a scenario in the editor and click on the little play icon on the taskbar, it opens it up in-game turn 0 and you are able to get some units and play the mission out. I am not 100% sure you will get all the scripts right that might be checking awards/tokens form earlier scenarios or things like commendation points, but missions should be perfectly playable this way. But don't take my word for it, try out a scenario from one of the core campaigns like that to make sure it will work for you, as I personally didn't attempt it like that.
I guess it works since this method allows map designers to easily playtest single missions like that.