How to create child cases from custom case numbers in Salesforce?

Let me explain it precisely. We are supposed to create cases for our customers through vf page for quality issues related to their products. We have different functional areas such as Design, Quality etc., For ex, customer A has a issue related to his product design, we create a case, say the case number is (QR-121). That issue is resolved and case will be closed. Customer has another product which has the same issue with product design in the near future. so, we create a case (QR-145). Since there's a similar case resolved earlier, we want to use that case as a reference. I've a look up field to case to add the reference case (QR 121), but when I try to look up for QR 121, it obviously won't show any records, as the lookup is associated with the standard case number. How can I be able to add reference cases by it's custom case number (QR 121) instead of the standard case number?