Hi Bin,
I do not recommend this but you can try.
1. Extend Inbound Delivery and enhcance beforesave event
2. In beforesave, query PurchaseOrder-Item and assign extension fields with delivered quantity
FYI:
There is another Purchase Orders work center under Inbound Logistics.
There, you can un-hide Delivered Quantity in Item table using KUT (Adaptation Mode).
Best Regards,
Fred