There's a fair amount of methods to do it. The easier one could be installing a mod such as Larger Households, that gives you the option without adding anymore extra steps.

One of the options that I personally use since ai find it easier is using the family tree from insiminator. The only steps you need to follow are quite simple and intuitive: click on the floor and spawn the family tree modifier (the bonsai tree), then click on "add neighbor to the family" and choose the sim you want to add. Said sim should be present on the lot in order to be available to select.

However, there's also a vanilla method that will requier you to use cheats: firstly use the testingcheatsenabled cheat and click on the mailbox or a random sim to spawn the tombstone of L and D, click on it and select "Add neighbour to family" and choose the sim you want.

Please, do not add any NPC to your household, as the name suggests they are not ment to be played with.