Thanks for your reply to my post.
Yeah! I didn't like the answer but at least now I know that there is no other way to do it.
For work-around, I am now coding fix using custom NER call to do following:
When I am in update mode and last status is 520(on entry only), I call custom NER and update...
When I create Direct Ship Order, it creates Purchase Order successfully. When I go into detail line revision, it gets next number for purchase order and copies the line number from sales order line. (S9 and P9 is custom order type similar to SD/OD)