You have several options for this. If the data is in another table, ( Not the same table/ different table occurence ) you can import it directly. Just select the current file and the correct table.
If it is in the same table, you can export it to another file and then import it from the exported file.
You should have it match on an ID field so that you update existing records, and then have it import those that don't match. You will see these options on the import screen.