Hi avr001
Extensions are loop mounted squash file systems. They are read only and can not be edited.
Do this:
busybox cp -f /tmp/tcloop/openbox/usr/local/etc/xdg/openbox/menu.xml /usr/local/etc/xdg/openbox/
Now edit /usr/local/etc/xdg/openbox/menu.xml. Add usr/local/etc/xdg/openbox/menu.xml to your backup.