Once a book has been generated, a new characters cannot be added.
If your book is currently Paused, you do have the option to transfer your purchase to a new book and add your new character. Your prepaid printing and shipping will transfer to the new version, so you will not need to purchase the physical book again.
From the current book’s Order Status, click How do I make changes?, then select Create new book. You can then upload the new photo for Will during the initial character setup before generating the new version.