Read carefully the section about updating matching records:
Note: importing cannot delete records. After import, the found set contains only imported/updated records.
In the Import Field Mapping dialog select "Update matching records in found set", then check the box to "Add remaining data as new records". Make sure your key match field is defined then click Import. As described previously, on screen will be the imported records. Choose Show All Records (Ctrl-J) to see all records.