Remember too that if your existing site is based on "flat HTML pages", you will need to "un-learn" what you know about HTML.
While zencart will BUILD its "pages" using HTML protocols, it does so "dynamically" using PHP scripts and a MySQL database.
Each "page" does not exist until it is "called" by the site user(s). Zencart will cobble together all the elements a "page" needs (from data in the database, and resources like images, and other code files), and then deliver that page to the viewer's browser screen.
When the viewer clicks a link, or performs an action, the "page" they are on evaporates forever, and a new page is built by the software.
This DYNAMIC nature of zencart is common in all modern "CMS" websites, so weaving zencart into your existing site is technically very challenging.
HOWEVER... zencart is quite capable of taking your existing layout and content, and incorporating it into the zencart structure, so rather than trying to weave zencart into an existing HTML site, it is FAR EASEIR to weave an existing HTML site into zencart...