For this to work you have to have the Magic Llama cheat (elsewhere on the site) and six Sims already in your household.

1. Invite someone to your house.

2. Wait until they come over.

3. Hold LT on them and then press A on them.

4. It will say something like "put in household" and then they will be included in your household.

Note: it may not be possible for switch Sims until they die, but you can use the trigger age cheat (also elsewhere on this site) until they are completely gone.