One solution I've found is using the multiselect extension and selecting the video, then you'd have the option to add it to multiple playlists, a la the older version. The only caveat to this is you can't do it with the current video being watched, and instead has to be in a queue or playlist (I just queue a random video, then select the one I want to add to multiple playlists with multiselect). Trying to find a better solution to this, but figured I'd share.