Title. I've seen people complaining that you can't add interior doors to buildings, but I'm wondering if that's actually the case. It seems to me that if you build a room at a time, you might be able to get interior doors that way. Basically, snap, say, 2 floor panels together, build walls, including doors where appropriate, then simply snap another piece of floor on the other side of the door. In other words, instead of building the entire building and adding interior doors after the fact, build your house one room at a time, adding interior doors as you go along. It's a little more annoying, as you have to either know what you want beforehand, or build to see what you're working with, then demolish or store what you've built to add interior doors, but it would be a way to get them.
The catch, of course, is that you need to be able to snap floor tiles to both sides of the doorway, and I haven't messed around with the settlement system at all, so I don't know if that's possible. I'd happily test this myself, but I'm stuck at work for the next 5 hours, so I figured I'd ask instead. Hopefully someone either knows the answer, or can jump in-game really quick and see. Testing would be simple: drop a floor panel, put a wall with a door on one side, then try to snap another floor panel to the side of the first floor that has the doorway.