Hello Georgi,
What is your desired end goal in using Q-in-Q? Are you stating that the vlan you want to extend happens to already exist along the intended path for a separate purpose?
In my experience Q-in-Q is great for tunneling vlans across a network without having to define them along the path. There was a Juniper Facebook post displaying the following example which helped me out in my implementation. Hopefully this will help you out as well.
~J