As the question states, is it even possible? I haven't found a single way to move an issue back to the backlog from the Kanban board.
Oops, I just noticed that you have an overview of both the board and the backlog when your are on the page called "Backlog". You can there, drag and drop issues from one to the other.
Although, I am still missing the possibility to click an issue in the backlog and sent it back to the backlog from that view.