With later versions of Dynamicweb "stock location" has been introduced. In the video linked to further down, you'll see how to obtain the data structure for a source file, how it is edited and followingly used to update table EcomStockUnit.
By exporting the file we automatically obtain the structure we also want for import.
Now, several Ecom tables have been updated and applied with an ID column. This has been introduced to achieve better performance on the database. However, as this ID column has auto-increment, it will contain a value that does not identify the row from user perspective. To work around this you should notice - in the video presentation - the changing of the key that we use for recognizing a row. In the presentation you'll see the key being changed from "ID" to ProductID+LocationID when creating the import job.