Can you add a wild card to a completed book?
According to Pagat.com:


  A meld cannot contain more than seven cards, and a partnership is not allowed to have two incomplete melds of the same rank, but if you complete a pile, you can then start another meld of the same rank.


So not only can you not add a wild card, you cannot add any card. You'll need to attach it to another meld in progress, or make a new one.