If you want to keep the current part of your site as is, the headers can stay as you show them there.

However, the Zen Cart part will need the graphic look laid onto ZC structure (simple to do as described above) in order to function smoothly.

The idea of a hybrid site with ZC and HTML sections raises other issues, like session id problems, which others can address better. It is apparently possible but is not a recommended method. It may work to integrate the HTML pages into the ZC ez-page structure... though I can't help with technical details of that.

Good luck!