Export the gradebook as an xml file. Choose or create a course. Make sure that all students enrolled have the ID number field filled out. Create or edit an activity (one that will produce grades) and make sure that the ID number (Under "Common module settings") is filled out. Go to a course and then [Administration ► Grade administration ► Export ► XML file] . Make sure that the activity that you edited is show as selected under "Grade items to be included". Click submit. If you see no preview rows with student names then double check your settings. Click the download button. Edit the file. Open up the xml file in an editor. Change some grades around. Make sure that at least one grade is just a dash "-". Upload the xml file. Go to a course and then to [Administration ► Grade administration ► Import ► XML file] . Upload the edited xml file. Ensure that there are no errors.