The simple answer is just to use FTP or WebDAV. You may need to turn the FTP or DAV server on in etc/zope.conf, but once it is enabled, you should be able to access your site over the FTP or DAV port (set in zope.conf), log in with your Zope or Plone username and password (note: Plone usernames will only work if the path you are connecting to is inside your Plone site), and drag-and-drop content in. Folders should become folders in plone, images should become images, and HTML files should become Pages.

If you have limited amount of content, you can also just create the folders and pages in Plone yourself, and copy-and-paste the content in.

However, that may not get you exactly what you want. Most flat-file sites in HTML contain a lot of formatting and navigation, the equivalents to the navtree and tabs and general styling of Plone. You probably want to move to a CMS precisely because you don't want to manually maintain all that. Unfortunately, Plone has no ways of knowing which parts of your HTML file are real content and which parts are navigation. If your content is poorly structured, e.g. it contains a lot of <font> tags or other non-structural markup, or it depends on particular CSS styles being available in an external stylesheet, it may be hard to make the content look like it fits into the rest of your site.

The long answer, therefore, is that you need to ensure you have structural HTML - markup that only describes your text with <p>'s for paragraphs and <em> for emphasis and so on, without navigation structures or visual markup. Then, the import procedure above should work.

To get the same look-and-feel as your site, you will need to develop a Plone theme (aka "skin" or "style") that themes the whole Plone site to be in line with your desired design. There are several how-tos and tutorials on this subject.