This can be done with some extra setup.
You can import your data into a temporary import table where the Key field is your own PK. The based on a relationship where that is the parent make a relationship to the real data field based on a reference field on the child side of the PK field.
Then make a summary total of the # of existing data records. Then either using a saved table to table import or a manual "copy these records to another table:, filer the record based on # of existing data records = 0.
That way it will never import and update existing records.
When you get the next XL file, you purge the temporary import scratch file data and start again fresh.
