Yes, Sitebuilder's OpenArchitechture will allow you to modify any changes with a third party HTML Editor such as FrontPage, Expression Web, or Dreamweaver (all supported by Applied Innovations).

However, since Sitebuilder is a PUSH application (meaning that the content is stored on its local database and it is published or replicated to your site) it will not be able to read or modify any changes made outside of it's realm.