The way we do it is to make a column in the Excel sheet called Publication source. Before import, go to Documents and enter your source.
We have a protocol that requires each of these documents to have at least one entry for location and if the source is used for import, then two file locations.
For Example:
Physical (Storage Location in General Tab) KIC Records, 3rd Drawer, Orange Habitats+Species folder.
Electronic (Other File in Other Tab) Z:\Biodiversity Data\Metadata\Catalogue notes\KIC\2001
So when you do your import, Choose 'Publication Reference' in the first screen along with everything else & there ye go!
Cheers now, Rob