It can be done via cheats (Nightlife req., I believe):
boolprop testingCheatsEnabled true
Then shift-click on the mailbox and choose Spawn--Townie & NPC Maker
(Haven't tried this in a long, long time, so it may be shift-click on a Sim rather than on the mailbox -- should be obvious by the options that come up when you shift-click.)
Or you can try this object which you can place on a lot and which does the same thing and doesn't require entering the cheat mode:
http://www.modthesims.info/download.php?t=216865Note: It can take the game quite a while (several minutes in real-world time) to fully generate the new character, so don't be put off or assume anything is broken if it seems that nothing is happening after initiating the Create option.
Also keep in mind that the more total characters in a 'hood, the slower the 'hood will load.
Or you could just create Sims and use Insimenator's options on them to "Make Me A Townie/Downtownie/NPC".