It is not possible to add a QR code to a kit. The reason for this is that we consider kits to be flexible combination of items that can be rented out together, but can also be rented out separately.
A good example is a kit containing speakers, subwoofers and full-range cabins. You will mostly rent them out together, but sometimes you want to rent them out individually.
You can however, assign QR codes to cases. In this article you can learn more about kits, cases, and accessories.