Magento绕过库存检查添加到购物车事件

I need to write a module that bypass the qty check when customer add the product to the cart (I have an ajax add to cart). So, any product ( also the out of stock one ) can be added to the cart. Only when i go to the cart page, magento show the out of stock message for that product.

Is it possible? Many thanks

GoTo Admin menu

System > Configuration > Inventory

Product Stock Options to

Set Backorders to Allow Qty Below 0 and Notify Customer

See my screenshot below

you can set individually from product configuration too