Stevesh is right... Zencart is designed to be your site, and you cannot (easily) embed zencart into "existing" html pages.

You CAN, however "incorporate" html pages into your zencart site - as long as you understand that, for the most part, the e-commerce functionality of the site relies entirely on zencart code, templating and database structures. You make your HTML pages look like Zencart pages, rather than the other way round...

I am busy helping hummingbirdbooks.co.uk to invoke a "simple" subscription-based website, and the methodology we have chosen (for various technical reasons) makes use of BOTH zencart functionality and set-up, AND a few "flat" html files.

If you look at the hummingbirdbooks.co.uk site, you'll see what I mean.