I was trying to do this as well - it is controlled in the administrator/menubar.php file -
Just write the link like this example
Look for the menu item you want to put it after then place it just above the last _cmsplit.
Note that you can substitute the first null with an icon that you want displayed with themenu item as in this example.
['<img src="../includes/js/ThemeOffice/backup.png" />','Backup', 'index2.php?option=com_dbadmin&task=dbBackup', null, 'Backup'],
Hope that helps - that is what I did and it works grand. You can also look in the files for the example of how to set submenus - very easy implimintation.