Custom Bill Type

It would be useful to be able to have custom Bill types. As the moment you can either have the bill as a draft or approved. However it would be useful for me to have a third category which is 'ready for payment' or something similar. this is because I have to wait for my customer to pay before I can pay my supplier, so it would be good to have an option that shows the customer has paid so now the supplier can be paid. I can't find a way to do this at the moment