Manufacturing control (Item level in Sales Orders)

Manufacturing control (Item level in Sales Orders)

We use Accounts, Quotes, Sales Orders and Invoices to control a manufacturing order-basis business. When a Sales Order is "Approved", the Manufacturing department starts to produce it. When Sales Order is "Delivered", we convert it to Invoice.

We need to control the number of Items in the Sales Orders that are already Manufactured. So, the right way to this is create a Custom Field in the Product Details section of the Sales Orders indicating "Status of the Item" (Pending, Manufactured, etc.) but it's not possible to include Custom fields in the Production Details section. 

Any idea to do this?

Thank you.

Regards,
Jesus