Change a product stock to onbackorder

Hi, 

I export my products and I want to change the stock value to onbackorder (not instock or outofstock)

How can I do that?

I used to use a different plugin and in "stock status" i could use the following commands

instock : It is available for anyone to order and pick it up right away
outofstock: no-one can order it 
onbackorder : anyone can order it  and it will be available after 3-4 days

I would like to do it that way as well because I need this stock status.

Right now I see a check box which is only in stock or out of stock. I don't use quantities .
I have a column on back order, i use the value yes, but it doesn't change the message as it used to (available for order after 3-4 days).  I still have to check in stock or out of stock and I don't know how to make it show the last message that I need.

Any idea on how I can do it?

No. I thought that in woo commerce this is a standard value in stock status.