Hi,
You can add the default WH when you add items,
the filed is [DefaultWarehouse], and mandatory fields is [ItemCode],
you can find the template in
C:/・・・SAP\Data Transfer Workbench\Templates\Templates\Inventory\Item Master Data\[OITM - Items.xlt]
about others WH, you can add with the template of [OITW - ItemWarehouseInfo.xlt].
Good luck.
