The full list of Import tags are available in the Endntoe help (try index search, Import file where I found this):
%W
Database Provider
However, unless the database outputs this info in their export file, I find it easier to select the newly imported group (automatically generated temporary group after copying or importing) and globally edit the field in question using the Move/Change field endnote command (under tools) to add or replace the Database provider field with the appropriate text.
relevant extracted Help section from EndNoteX5
When the import is complete, the newly imported references are displayed in an Imported References group. This is a perfect time to add a keyword to each imported reference with the Change Field command, or to peruse the imported data to make sure it imported as expected. Make sure to check references that contain extended characters.
The Imported References group is a temporary group that is replaced each time you import another set of references. It is removed when you exit EndNote (although the references remain in the library).