Scenario: You’re using the import wizard. For those that are duplicates, you want to add new information and update blank fields.
Solution: You can add new information on an import file in a couple of ways:

  1. If the added information is standard across all records, such as Lead Source, select the field on Step 3 (mapping) even though it’s not included in the CVS file.  Then on Step 4 you can quickly enter a value in the field and copy it to all records.
  2. Alternately, on Step 4 and Step 6 you can edit any fields on the individual groups with inline editing. This presumes you did not select auto merge or auto push on Step 5.

Lastly, you can configure your automation settings to select the correct values from the import file or from the records in Salesforce. However, this configuration relates to updating or changing existing values, not for adding values that aren’t already there or blank fields.