How to Add a New Sim to Already Existing Game?

You can also use testingcheatsenabled true, edit your sims with shift enter. Then, choose save sim. ( click the three dots at the bottom of the screen.) this will save the sim to the bin. Then make a new file, choose create a new household, then add pre made sim. Then click custom sim. Then, add other sims. You should then be able to replace the new family with the old family in the household