If you would like to allow customers to order items that are out of stock, you can do this by turning  off the inventory check in your "Catalog Settings" page.  Just login to your store admin, then click "Catalog Settings" and for Inventory Check choose NO then scroll down and click the button to save your changes.

This will cause the cart not to check inventory levels and it will allow customers to order out of stock items.

I'd suggest clearly explaining how out of stock orders work so customers are aware of when their items will be shipped, if you choose to do this.