See this thread about default furniture found in furniture.jar file.
This file is some kind of special .sh3f file that you can open and modify with recent versions of the Furniture Library Editor. Once changed, save it under a .sh3f file with the name of your choice and import it in Sweet Home 3D, where they will appear if you removed furniture.jar file.
Keep in mind that updating Sweet Home 3D may reintroduce furniture.jar file and you’ll have to delete it to make appear your modified furniture again.