You have two options to accomplish this.
The first option is to use external .html pages. Create or copy your old html pages in the file structure of you site. Create identical pages in Sitefinity and set them as external pages. Type the actual URL in the text box that appears below the option button. Note you cannot edit external pages in Sitefinity.
Your second option is to set Sitefinity to use .html
extension for its pages instead of .aspx
. This way you will be able to manage the controls and content of these pages. To change the extension, open your web.config and change the flowing element like this:
<cms defaultProvider="Sitefinity" pageExtension=".html" redirectToHomePage="true">
Don’t forget to set the appropriate ISAPI mapping for .html extension in IIS!
Please see Configuring IIS section in User Manual page 143 for more details.