Use add_submenu_page instead.
<?php add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function ); ?>
$parent_slug should be one of the following
For Dashboard: add_submenu_page( 'index.php', ... ); Also see add_dashboard_page() For Posts: add_submenu_page( 'edit.php', ... ); Also see Also see add_posts_page() For Media: add_submenu_page( 'upload.php', ... ); Also see add_media_page() For Links: add_submenu_page( 'link-manager.php', ... ); Also see add_links_page() For Pages: add_submenu_page( 'edit.php?post_type=page', ... ); Also see add_pages_page() For Comments: add_submenu_page( 'edit-comments.php', ... ); Also see add_comments_page() For Custom Post Types: add_submenu_page( 'edit.php?post_type=your_post_type', ... ); For Appearance: add_submenu_page( 'themes.php', ... ); Also see add_theme_page() For Plugins: add_submenu_page( 'plugins.php', ... ); Also see add_plugins_page() For Users: add_submenu_page( 'users.php', ... ); Also see add_users_page() For Tools: add_submenu_page( 'tools.php', ... ); Also see add_management_page() For Settings: add_submenu_page( 'options-general.php', ... ); Also see add_options_page()
Example:
add_submenu_page( 'tools.php', 'My Custom Submenu Page', 'My Custom Submenu Page', 'manage_options', 'my-custom-submenu-page', 'my_custom_submenu_page_callback' );