I just released a new plug-in that allows to copy the selection as a new piece of furniture into clipboard.
This plug-in is available at http://www.sweethome3d.eu/plugins/CopyA ... 1.3.1.sh3p

Quick user guide:
- Install plug-in,
- Select in the plan the walls, rooms and furniture you want to group as a new piece of furniture,
- Choose Tools > Copy as new furniture,
- Choose Edit > Paste to paste the new piece of furniture in the existing home or an other home.

I hope it will satisfy your needs. [:)]

Note:
This plug-in was updated to version 1.3.1 on August 28, 2024
This plug-in was updated to version 1.3 on November 18, 2020 to handle the copy of groups, standalone rooms (only if at a upper level), furniture elevation at different levels, texts and polylines visible in 3D
This plug-in was updated to version 1.2 on February 4, 2015
This plug-in was updated to version 1.1 on December 30, 2010