I´ve been asked by an coworker whether it is possible to post snapshots using Anywhere to already exisiting threads. It seems to me now, that unfortunately posting snapshots only goes along with creating a new thread.
Adding snapshots to existing discussions, or to comments, is not currently possible. It's something we may consider for the future, particularly around adding to comments. If this is something you're interested in, I would encourage you to create an Idea here for others to weigh in on also.