wangzh2k, you most assuredly can design a non-standard flange with Appendix 2.
As for your ASME (no longer ANSI) standard flanges, while I am not sure of the exact reasons, it is well known these do not meet Appendix 2 design. I believe that at least part of the reason is they are typically much over-bolted compared to Appendix 2 design.
Try a site-search, this has been discussed before.
Regards,
Mike