As long as you have the original web root folder in your My Webs or My Websites folder in My Documents you can just open the site and edit it to your hearts content.
One easy way of adding a new page is to open an existing page in the web root folder, say your index page for example. Click on File, Save as, and give it a new name. Add hyperlinks in the other pages to point to the new one you just made by saving the index file as a different filename and you're off.
This preserves any layouts, themes, navigation structure and so on without you having to manually configure everything on the page form scratch.
If you have a navigation bar set up you can update it to inlcude the new page.