When creating a reservation, is there a way to include ordered quantity in the available quantity?

When creating a reservation in the Inventory module, is there a way to include ordered quantity in the available quantity?

Ex: On-hand: 100,  Reserved: 80,  PO: 500         and I would like to create a reservation of 50. 

Currently, only (on-hand - reserved) build the available quantity. So, is there a way to tell the system the available quantity = on-hand + ordered - reserved?

Note: We are not using Project Manufacturing or Order Entry.

Thanks,

Jacinthe