Add Sims during gameplay

Yet it had worked for me...
Well, we will use cheats!

Start by creating your sim, then move them into another house. Go back to your game, and once it's loaded, press:

Ctrl + Shift + C (this will open a dialogue box.)

Then type: testingCheatsEnabled true

This code mostly works with the mailbox (you can disable needs, etc. Anyway)

Invite the character you want to add to your home, press "Shift" and click on them.

Select "Add to household."

In the pocket!
Enjoy the game! :)

I should specify, I would like to add it in the same house...

Once the family is created, just take it from the library and click on a inhabited house; the two families will then merge.