Allow back orders

It would be helpful if we could indicate on a particular item that back orders are allowed and if we have an anticipated availability date, we can put that in as well or say unknown. The customer would not need to pay for the item, but would have to have a credit card on file so that we would charge them when the item arrives. There are some items we would do this for and others that we would not want to do this for because what we have in stock is all we are going to ever have.