Currently only Leanpub admins can add books to bundles.
Please note that existing customers of the bundle do not get any books that are added to a bundle. There are a number of reasons for this, but the most important one is that we do not want to unilaterally add content to people's Leanpub libraries.
Typically, we recommend that authors create a new bundle with the books in the original bundle, and then add the new book to the new bundle. (Since there are more books in the new bundle, we also recommend you set a higher minimum price for it than for the old bundle, but that's just a suggestion.)
If you have a book you want to add to an existing bundle, just let us know which book you would like added to which bundle, and we'll do this for you or follow up for more information. Please email us with the required information at firstname.lastname@example.org.