Attach read-only dmg file using hdiutil
hdiutil attach -owners on /path/to/your.dmg -shadow
Modify your DMG. Use the command line to add, edit or delete contents – doing it in the GUI can have weird side effects or permission issues. Additionally, you may need to sudo the commands
Detach mounted drive by below command or Eject it from Finder
# fill disk number at
hdiutil detach /dev/disk5
Converting/Writing a shadowed dmg into read-only DMG
hdiutil convert -format UDZO -o /path/to/new.dmg /path/to/your.dmg -shadow