Sometimes, clients can come up with some pretty creative ways to handle their inventory … It was the case for me with a customer who was using WooCommerce.
I had to add a third option but the stock options was not customizable. I ended up deleting the native dropdown of stock options and I implemented a custom field.
And then I followed this tutorial to create a custom field : http://www.remicorson.com/mastering-woocommerce-products-custom-fields/
Someone on SO proposed an other solution based on my approach but my contract was already done with that client by the time he answered, didn’t get the chance to test it out.