In hindsight, we probably could have several regular people infiltrate the Imperium instead as long-term agents. We don't have to make space marines do that because it might not be practical for a long time.
Making a background for them would be very easy. They were "refugees" from one of the planets attacked in the war. So we could in theory have millions of spies from "migrant ships" trying to escape destruction. With the chaos of the war, there could be plenty of opportunities for agents to get them into middle management systems. Nothing crazy like becoming Planetary Governor (although that would be impressive), but have access to a lot of intel.
A good backstory for our spies is that they are a blend of a PDF legion and several relatively important government officials... along with several aids connected to them. Their Planetary Governor demanded a full evacuation of the planet and abandoned the world to it's fate (typical callus behavior for them, so nothing that suspicious). Shame they died during the "evacuation" but there are "family members" who could fulfill said duties. But they will have to rebuild their power base from scratch.
At least the Planetary Governor had several other ships that got "lost" in the chaos of the conflict. (A perfect way to bring more infiltrators at later dates.)