Thank you for asking this question in the Support Community. At the time of this post, ADTRAN does not support Q-in-Q tagging. I recommend you contact your ADTRAN sales representative to submit a feature request for AOS on this topic.
Please, do not hesitate to reply to this post with any questions or additional information. I will be happy to help in any way I can.
I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post to unmark it and select another in its place with the applicable buttons. If you have any additional information on this that others may benefit from, please come back to this post to provide an update. If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.