Sorry, I do not know more than what is in the Wikipedia page I linked to. I looked at it because I did not even know what Hangul was, nor the the HWP format. Presumably, the specification in on this page: http://www.hancom.co.kr/userofficedata.userofficedataList.do?menuFlag=3, or linked to within this page, but I do not know Korean at all, so I can't help you further.

Edit: Your Mother provided the link while I was fighting with my computer and Internet connection over mobile network, so (s)he was the first to provide the link I have above.