Google has released an API for Google Sites that lets you create or edit pages, upload or download attachments, monitor the activity of a site programmatically. The API could be use to create a new interface for Google Sites, to upload files from other sources or to migrate your data.
Google's Data Liberation team built a Java application for importing and exporting Google Sites. The application lets you export the pages from a site and all their attachments to a folder.
"The folder structure of an exported site is meant to mimic the Sites UI as closely as possible. Thus if exporting to a directory "rootdirectory," a top-level page normally located at webspace/pagename, would be in a file named index.html, located in rootdirectory/pagename. A subpage of that page, normally located at webspace/pagename/subpage, would be in a file named index.html in rootdirectory/pagename/subpage. Attachments are downloaded to the same directory as the index.html page to which they belong," mentions the user guide.
You should only enter the domain name if you use Google Apps. "Webspace" is the name of your site: http://sites.google.com/site/sitename/.
Unfortunately, you can't use this tool to import HTML files to an existing site. The importing option is only useful for the sites exported using the same application.