In theory, you can create new types of nodes (for Behavior Trees, it's not really needed, because you would create a different kind of structure). Actually, you can create different tree structures that are not Behavior Trees anymore (e.g. a Dialogue Tree), which are far more useful than creating another node for a Behavior Tree. Dialogue Trees are really interesting, because they use a very similar structure to a Behavior Tree, and you can use the same Editor (or better, a slightly modified version of it) to edit the tree.
As much as I would like to dive into these topics, the main focus of this book is on AI, and so talking about Dialogue Trees is outside the scope of this book.