When you edit the body of the message with Dreamweaver it should open up your new page and allow you to add in whatever you would like. When you save it the page should go away and the Universal Email interface should open back up with the page previewed.
To test this out a little further please try the following. On a new site that is properly defined create a new page and save it, then apply UE to the page and craft the custom form for the body of the message. Please post back and let us know what your result is on this, if it works ok here then it may be an issue with the site you are working in, or a problem with the Universal Email instance you have on the page.