There’s a great post on the message boards about adding products to a standard price book using Ron Hess’s Excel Sforce Connector.

The highlights are below but you can also click here to read the full thread.

To add products to the Standard Price Book, you have to add entries to the 'PricebookEntry' table.

The table contains three id fields:
Price Book Entry ID
Price Book ID
Product ID

The Pricebook ID is of the Standard Price Book which you can get from querying the Pricebook2 table. The Product ID is of the Product you want to add and you can get that from the Product2 table. The Pricebook Entry ID value is 'new'.

Fill out the rest of the table as needed (List Price, Active, Use Standard Price, Currency). If you need a product with 2 currencies, add the product again to the PriceBook Entry book and just change the currency field.