If you look at the source for the function, you can see that the arguments to the function are passed directly through to get_pages(). That function supports a post_status argument that can be an array of statuses. So to do what you want all you need to do is pass the post statuses you want into a post_status argument of wp_dropdown_pages():
wp_dropdown_pages( arrray(
'post_status' => array( 'future', 'publish' ),
) );