Mac OS Add Login items to all user accounts at once

1. Login as local Admin.
   
2. Copy your application to Macintosh HD:Application folder.
   
3. Open System Preferences -> Accounts -> Login Items and drag/add your application there.
   
4. Open User:Library:Preferences, there you will find a file called loginwindow.plist copy it to Macintosh HD:Library:Preferences

1. Make loginwindow.plist in Macintosh HD:Library:Preferences read only to all users by right click -> Get Info
   
2. Restart
   
3. Enjoy!