I can suggest you a dirty workaround, waiting for a better solution.
In Sweethome3D put your car in a new model (only the car), click on Export to OBJ format.
Retrieve the file.obj. Open it in an advanced editor such as Visual Studio Code.
Then follow these steps:
- In place of the first "g " tag put something like “g_your_car_name”
- Cmd-F to open find.
- Paste the string "g "
- Alt-Enter to select all of the instances of the string on the page.
- Cmd-L to broaden the selection to the entire line of each instance on the page.
- Delete/Backspace to remove those lines.
- change the first g line to “g your_car_name”
Save your file as car.obj and reimport it as furniture (from the menu, Furniture, Import Furniture) give it a new name and insert it into your catalog of objects.
Satrting from your car will be made of only one object when adding it to your home model.