Adding fields to spreadsheets

IS there a way to add in your custom fields to the spreadsheets you upload for your product imports? We have part numbers we would like to add to each product. Rather than tediously going into each product and adding the part number is there a way to add the custom field to the spreadsheet we import so we can add the part numbers to all of our products at once?

The page I need help with: [log in to see the link]