This is possible, but there are three big parts in your question:
1. You need to find a working addon for reading QR codes, try this link or search the forum:
construct.net/en/make-games/addons/414/qr-code
2. If you want to make changed to JSON, you need to parse it, see the official JSON example in C3.
3. To send data to a server you need to use AJAX and possibly PHP/MySQL, there are lots of tutorials available here:
construct.net/en/tutorials