Add Assigned Letter Field to Transaction Import

When importing transaction data from offline or third party sites, I would like to have access to the Bloomerang field under "Acknowledgements" called "Assign Letter" that shows up when you manually enter a donation. If I could add that field to my spreadsheet prior to data import, that would save me so much time from having to do manual work after the data is imported. In more than 50% of cases when I import data I need access to that field to avoid having to return to each individual record to assign the letter.