The Html content gets its data from the db. I think the only way you can make it display the contents of another page is to go into source view with the editor then add an iframe with src pointing to the page you want to show.
The HtmlFragment include module was something I threw together quickly for someone. It includes html from a file in a specific location which has a fragment of html without the html and body elements since that is already on the page. I built it specifically for Rene Mayrhofer so he could show his publications
which are in a fragment file put out by some other system. Its a very simple module with a very simple purpose.
Hope it helps,