You can just put an "online store" link on your current site. The difficulty with integrating an HTML site with a PHP e-commerce site is that if your customer leaves the cart and spends much time on the HTML pages, their session will time out and they will lose whatever is in their cart (unless they are logged in).

There is a Sidebox Anywhere mod that can let you show some Zen Cart content on the HTML pages.

The best solution long-term is to make your ZC home page and some other pages replicate the current contents, and have one system for the whole site.