Hi Jennifer,
Applying a default warehouse to items is simple enough, just use the normal Item Master Data template: C:\Program Files\SAP\Data Transfer Workbench\Templates\Templates\Inventory\Item Master Data\OITM - Items.xlt
In it you will find the DefaultWarehouse (DfltWH) column. Just fill this column and the ItemCode column (and delete all other columns), and you are good to go.
If however, you also want to move all stock to that warehouse, you are in for either a lot of clicking and typing, or you will have write a small application to do it for you.
Regards,
Johan