I don't understand. You can already do this, by building space stations and grouping them together in a fleet. For example, I have a fleet called Cloud City on Venus. I add orbital habitats on it whenever I have surplus minerals and factory production. But for every practical purpose as well as visually, it's a single giant space station. You could even rename the individual "ships" to things like Hudson Arcology or Equatorial District so that even in Fleet window it'll look like a city instead of a list of OHB 001, OHB 002, OHB 003 and so on.
Unless I misunderstood your post and you meant something else?