You will need to use Power CMS to integrate CMS features into an existing site. Have a look at the tutorials on the WA site to show you how to do that - there are a number of steps you need to follow to set it up.
You then have to create content regions for each page and add your content from the exisitng web page to the Power CMS system.
We do this quite often for client sites where they initially have a brochure style website and then later on want to add content management to the site.
Hope this helps you.
