NPC Sims cannot be chosen by player. The game will place the NPC's in your apartment.

From what I've been able to tell the doors if external need to be enclosed with a fence and a gate. The public markers may need to be placed in the yard as well as up on the porch, if apartment has a porch.
Options for internal doors.

The option does not show on external doors.

Options for external doors inclosed in a fence with a gate.

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As a quick refresher, here's the instructions on creating/editing an apartment on a regular residential lot:
1. Have testingcheatsenabled on
2. Buydebug on
3. restrictbuildbuyinbuildings off (so you can edit/create on the lot)
4. Be in Build/Buy mode.
5. Have both public and hidden markers placed.
6. Use Shift+Ctrl+click to create an NPC Door.
-- This can be done on all doors leading to hidden "rooms" that are facing public spaces.