You can allow ordering of items even if the inventory levels indicate that the product is out-of-stock.  In order to allow this, you must set the option in the Preferences for Product Stock.  This tutorial will guide you in enabling the out of stock product ordering in the Prestashop 1.5 Administration Panel.

How to allow ordering of out-of-stock products in Prestashop 1.5

1. Login to the Administration Panel as an Administrator
2. Hover over the menu bar where it says PREFERENCES, and then click on PRODUCTS in the drop down menu. Scroll down the list of options and look for Preferences: Product stock.  Select the Allow ordering of out-of-stock products option in the menu as shown below:
   
   
   
   
3. Select YES to enable ordering of out-of-stock products, and NO to disable the feature.  Note that the ADD TO CART button will hidden if option is disabled and product is out-of-stock
4. Select the green SAVE button in the top right hand corner of the screen to save your selection

To allow the ordering of products that are out-of-stock the option must be enabled in the Preferences for Product Stock.  Remember that this may cause issues with shipping times and promised delivery dates.