Thanks for the answer
I can certainly select the window areas and define them as separate objects even though there's no hinged canopy in this case. However, I was primarily wondering whether there's an easy way to slice with a knife to separate the window areas from the main fuselage. Since slicing with a knife places new vertices where we want the object to be separated it provides the right transition automatically. But then, for this job I'd need a "corer" to punch out the selected area and the knife function doesn't seem to work with multi-surface "knives". Otherwise I'll just go "insert vertices" until I get the ones I need, rearrange the surfaces and delete superfluous vertices and redundant surfaces afterwards.