Ive done it, not exactly "automated," but what Id call semi-automated. Export the resource assignments to a spreadsheet to use as your template. As you said, make sure you have Resource ID, Activity ID, and Budgeted Units. There will be other columns that P6 forces you to export also. Modify the assignments in P6 and reimport into the existing schedule. You can temporarily add columns, use formulas, or change colors in the excel file, but make sure you revert back to a plain file before you import. Also, its generally easier to make the assignment in the activities view and then modify in the spreadsheet.
Some important notes: dont change the column headers. dont change the second sheet that has user/database information. dont use any formulas or fancy formatting when you import. dont delete any columns. Make sure you have a backup of your schedule before you import. Make sure the correct schedule is open when you import, and make sure the excel file is closed. Import only one "topic" at a time... dont create a bunch of new activities, add relationships, and resource assignments in the same bulk import... do that in 3 successive imports.
In terms of rights, you have to have the ability to import. They have tried to separate the rights for importing from an XER vs a spreadsheet, but in my experience you need both rights.
