hello ,
I am not sure if I understand your question , better if you post a screenshot illustration , but if you want to mirror a group of walls you can select them then copy and paste the new pasted walls will be now marked as selected , just right click and flip them horizontally or vertically but be careful not to lose selection by clicking away .
another way by using "copy as new furniture " plugin http://www.sweethome3d.com/plugins/Copy ... n-1.2.sh3p select walls you want and copy as new furniture then paste and flip in the direction you want .
I hope it is helpful .
Regards