Associate data with existing items

I would like to add data to pre-existing items using the CSV importer. Does anyone know if this is possible? If not, what would be the best (easiest, least time consuming) way of making this happen?

Example: I have one hundred images in my collection and uploaded to Omeka with only 'Filename' tagged. I would like to associate five new fields of data (ie. 'Description', 'Date', 'Creator', etc.) with the existing items. My data is all prepared in a CSV file but I don't know how to automatically associate<\em> each of the new items with the pre-existing items.