[Help] Custom interstellar empires

Thus far, I've found Stellaris to be a tremendous amount of fun to mod and customize and reshape the galaxy in my image. Except for one small, persistent irritant that's been a barrier to forging a certain galactic scenario that I've been working on. I cannot, for the life of me, figure out how to initialize multiple systems, or even just worlds, as belonging to a single custom empire (human or AI).

I feel like it's something exceedingly simple and obvious, likely a simple matter of getting the right syntax, but I just can't grasp it. If anyone else has managed to pull it off, I'd be very interested in how they did it.