No, iWeb is not able to import HTML. iWeb is a sort of page layout program that uses a proprietary format until you are done creating a page, and then it exports and translates that page into HTML. It can't do things the opposite way.
In fact, though there are several easy to use WYSIWYG Web site creation programs for the Mac now that don't require that you learn HTML, most operate this way.
One exception is:
KOMPOZER (free and open source)
Kompozer is a very impressive application. It is a bit more complex than most other Web site creation tools for ordinary (non-professional) users. In fact, it is almost as powerful as a professional-level Web site creation tool. But there are several free tutorials to learn how to use the program, and it isn't hard to learn to use. You can easily migrate your existing Web site into Kompozer because it doesn't use a proprietary format, it will simply open your HTML-based Web site.
Review page: http://www.download.com/8301-2007_4-9714054-12.html
Download page: http://kompozer.net/