Hey so I was having the same problem and I figured it out. If you are in the manage world screen then select the household that you want to merge the pets with. In the bottom right hand corner (next to the play button) there’s the three dots, press the the dots and three other buttons will appear. Select the button that says manage household. A screen will appear and show you the household and give you some options in the bottom right hand corner again. Select the button that looks like a pencil to edit, add, or removed sims in CAS. Now it should take you to CAS mode. In the bottom left hand corner you can add a sim. Once you press that button the little menu will appear above it and you can add a sim from the gallery. Now you can select the household you wish to place right from the galley and they will appear.
