This is related to Threaded comments feature
However threaded comments aren’t there yet and I want to propose a UI feature that could later be used with threaded comments but now will be useful by itself.
I propose to add a button in the comment view (where report or delete comment buttons are now), which would be a “Reply To” the comment button. When user clicks it, the front end automatically inserts the mention of the comment’s author to the new comment input area and also quotes the comment text. This is similar to how comment replies work in VK.com.
When threaded comments feature arrives, this button will also select the thread for the comment.