Available to Promise / Rescheduling

Hi

We are running into a couple of issues with our setup in ATP.  In our current set up when we create sales order the ATP runs and checked the following

1) is there inventory - if so set the availability date based on this

2) is there an open PO - if so set the avail date based on this

3) Lead time - set based on this

the issue that we are running into is if our PO"s expected delivery date is longer than our lead time for that article.  In that case the ATP check looks at he lead time date to set the avail date.  Since our articles are specialty articles we cannot just send a new PO and receive it within the lead time

what we would like to do is the following

1) is there inventory - set based on this

2) is there an open PO (reguardless of the delivery date)

3) lead time (ONLY if NO PO exists)

thank you

we are currently on Retail SAP ECC 6.0 Linus, Oracle