Unfortunately you can't use the Excel Add-In to update fields at all. It's great for export-import but that's the full scope of the add-in. Depending on the number of test cases you're working with your best option in the UI would be to use a Custom List and bulk update that way (the Agile Central Labs "Bulkier Update" tool is awesome). Otherwise you'd have to work it out through the Web Services API. Hope that helps.