See:
MDLQA-8106 A teacher can copy and paste grades from a spreadsheet into the gradebook
https://tracker.moodle.org/browse/MDLQA-8106
I passed this test, but I had a question about uploading grades from the spreadsheet that matched the grade in the gradebook. In my tests, grades that were the same in the gradebook and in the OpenDocument were replaced. As an example if there is a dash in the gradebook for an assignment and in the OpenDocument that is being pasted in, the grade changes from a dash without an override to a dash from a grade override. This doesn't change the score, but can have a negative effect if you wanted to upload a few grades from the spreadsheet and then grade the rest from the activity.