Is it possible to add a QR Code while building the product? Either with a locked URL or have it editable. All I can see is the ability to add a new qr element but then the user can drag it around and destroy the design. 

Well, a locked URL you could do simply by uploading an image layer. There is however currently no way of adding a qr-code-layer in the builder. I suggest that you head on over to the feature request forum and add your idea there.