Nice start! [;)]
The easiest way is probably to add the model you want to change to the plan, select the 3D View > Export to OBJ format menu item, choose Export selection when prompted (you may also find the OBJ files at https://sourceforge.net/p/sweethome3d/c ... swap-cc-0/ ).
Then import in Blender the exported OBJ file with File > Import > Wavefront (.obj) menu item, modify your model and reexport it with File > Export > Wavefront (.obj) menu item.
Finally, reimport the generated OBJ file in Sweet Home 3D with Furniture > Import furniture menu item.
If the model is partly visible in Blender, reduce its scale because Blender works in meters and Sweet Home 3D in centimeters.