Generate Purchase requisition against internal requisition ?

I have a requirement to Generate Purchase requisition against internal requisition?

As a business case, When user will create a internal requisition and quantity is not available or insufficient then system should generate purchase requisition to fulfill the quantity, pr will convert to po manually and do the put away to increase the on hand. Once the on hand is available, the internal requisition will processed further for picking and shipping.

If anybody has an idea or any document to achieve this requirement ?