Adding link to dashboard sidebar, nested under custom post type

This should do it.

function register_additional_page(){
    add_submenu_page('edit.php?post_type=tree', 'Bulk Process', 'Bulk Process', 'activate_plugins', 'tree_bulk_process', 'render_bulk_tree_process');
}
add_action('admin_menu', 'register_additional_page');

And the function that renders the page.

function render_voucher_list_page(){
    ?>
        <h2>Hello World</h2>
    <?php
}

Change post_type=tree to your Custom Post Type, Bulk Process to the menu title, activate_plugins to your desired user capability, tree_bulk_process to your page ‘slug’ and render_bulk_tree_process to your custom function that renders the page.

Hope this help