Let's go under the assumption that you develop your content in FrontPage or Dreamweaver. This should be done in a folder other than the one used for your RoboHelp project.
As you complete pages, you can import them into RoboHelp, or you can wait until you compete them all.
The first page of your content should have the same name as the first page you defined in RoboHelp. When you import it, you will overwrite the original file.