As far as I know, preinstalled furnitures cannot be edited (or even found..).
Instead, furniture libraries can be downloaded from SH3D-site http://www.sweethome3d.com/importModels.jsp.
If you download for example 3DModels-Contributions-1.6.4.zip and open it with furniture library editor, you at least get an idea what can be done with editor.
Do your modifications, save edited file (filetype is .sh3f), double-click it and follow instructions to install modified library.

This is not exactly what you wanted but maybe it can help somehow.
Seppo