hi,
As per the standard SAP it will never do. However SAP has given some exits which might help to achieve this. Check out the exit save document prepare , set out some rules to redetermine the item cat. The does the item cat/schedule line determination first and then checks availability.
I am not very sure if this exit will work , never seen this kind of req in none of my projects, but you may try.
You will find all the user exits in the IMG system modification activity, Go through that doc and check with an ABAP.
Before you do any kind of ABAP changes do consider the cascading effects of it in other order types and the cut over spillage of the change.
regards,
Sayantan