citymajor as atreya33 says this can be done by saving your latest versions of your households to your library to place in another save. If relationships are lost you can use the cheat cas.fulleditmode to link them again in CAS. (Console players can use this cheat without disabling console achievements because it doesn't require testingcheats to be enabled.) I've done this before with families spread across a few different households and it was just a case of adding them to the original household, linking them again and moving them out again. You may lose career progress though, that's happened to me but there are cheats for that too, to get them back to their place in their career.
