How to Add a New Sim to Already Existing Game?

Hi There;
You can currently do that just follow this:

1. While in game play click '...' in the circle and select 'Edit Town'
2. Click the Create a Sim/Family Button.
3. Create your sim
4. In Family library put her into the household of the man
5. Merge familys (him+her) and they will be in the same household now
6. Go back to game play and click 'save' if it asks.

Hope ive helped.
Cheers.