Add custom button to edit page of sonata admin bundle

As you know, sonata admin bundle comes with three buttons in edit page which are "Add new, update and delete". I can remove delete button with this:

protected function configureRoutes(RouteCollection $collection)
{
    $collection
        ->remove('delete')
    ;

}

But I want to also add "Send message to User" button in edit of UserAdmin. How can I do this? I can't find any documentation about that in sonata docs.