在注册新自定义帖子类型的功能上，可以将此CPT设置为现有菜单项的子菜单。使用参数show_in_menu

例如：

register_post_type( 
    \'issue\', 
    apply_filters( \'wpit_register_issue_post_type\', 
        array(
            \'labels\'          => $issue_labels,
            \'rewrite\'         => $issue_rewrite,
            \'supports\'        => $issue_supports,
            \'taxonomies\'      => $issue_taxonomies,
            \'menu_position\'   => 5,
            \'public\'          => TRUE,
            \'show_ui\'         => TRUE,
            \'show_in_menu\'    => \'edit.php\',
            \'can_export\'      => TRUE,
            \'capability_type\' => \'post\',
            \'query_var\'       => TRUE
        )
    ) 
);