I'm not a programmer but for something similar needs I modify the program itself. Under linux!
Go to SweetHome3D-5.2/lib/ and You will find Furniture.jar archive which have the basic furniture. Modify this or replace with Your own.
Another way is to make a install script to put all the pieces in the right place respectively the custom furniture, plugins, texture etc in the path: .eteks/sweethome3d/