I'm confused by what you mean by "adding neighbors."
You don't add neighbors to an apartment. Story Progression generates them automatically to any doors marked for NPC use. To be able to mark a door, it has to be attached to a room that has the hidden room marker in it, and the area outside of it enclosed by a fence or hallway/vestibule/etc. that contains a public marker. Once the door is marked, over time, NPCs move in as the bouncers, mixologists, mailman and other service sims in your town.