Adele isn’t part of the 50 Songs Challenge. Recently there were 50 more songs added to the song pack, making it 100 in total. But the challenge was made when there were only 50 songs in the song pack, and the challenge list is still only the 50 original songs as listed here:
Not sure if there are any plans to expand it. Generally song covers not in the challenge go in the “Post Your Covers” thread, but it might be nice to have a thread or threads for the other song pack songs.