There are actually two first-party methods you can use to revive the “Add all to…” button for combining the contents of two YouTube playlists at a time, even if the source list isn’t in your library.
Both “old YouTube” (which says it’s “going away soon”) and the “disable_polymer” parameter trick (which isn’t said to be going away) have this feature that’s otherwise missing from playlists the user didn’t create. I’ll talk about the latter option because of its longevity, but you should skip step #2 if you’ve already set up old YouTube.
Follow these four steps to add the full content of any playlist to one that’s in your library.
- Go to a playlist page. The URL should look like
- Revert to an older version of the interface by adding “&disable_polymer=1” after the list ID.
- Click the three vertical squares at the top-right of the playlist sheet to reveal “Add all to…”.
- Select an existing playlist in the list or click “Create new playlist”.
The option doesn’t appear on the pages for auto-generated playlists such as “Watch later” and “Liked videos”, but it works perfectly well in most cases.