It depends on exactly what you're asking. if you want to add Zencart and link to it from your existing site, that's easy. Just install Zencart in a subdirectory (/shop, for example) and link to that directory.

If you already have products set up on your site and want to use Zencart as the back-end shopping cart, no. Zencart is designed to be a stand-alone application. The usual (and best) recommendation here is to re-create your existing site in Zencart.

Check the FAQ/Tutorials above for lots of info on installation, etc.