Thanks for reaching out about this and welcome to the Community.
Currently, you can do that only on the desktop app. You just need to hold Ctrl + A (Windows) or Cmd + A to select all songs in the playlist > drag them to Liked songs.
If you need a hand with anything else, the Community is here for you.
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“Music acts like a magic key, to which the most tightly closed heart opens.”– Maria von Trapp