That's not doable with out-of-the-box IMPORT statements.
You might want to look into SAP DataServices or the SAP HANA Enterprise Information Lifecycle Management Option.
Alternatively, you can approach this "manually" by mimicking the DSO loading and activation feature of SAP BW.
For that you'd load the data into a staging table first and then use a procedure to find all the keys that would be overwritten. Now you update the old records and insert the new ones - all in one transaction.
Again, there is no single feature that will do that for you - it's a part of your data design that requires development effort..
- Lars
