Yes, this is possible. How you configure it depends on a variety of factors - including what you currently achieve through your existing website, and what you want the "combined" site to achieve.
The e-commerce part should ideally sit in a sub-directory or perhaps a sub-domain, but this is not essential. However, if you put everything in the root directory you will need to adjust the navigation structure because your original site has an INDEX (or default) page and your webshop will also have an INDEX (or default) page.
If your existing site is well-established and attracts lots of visitors, and if it is well-indexed by search engines, it may be an idea to not interfere with it. Either put your e-shop into a sub-directory, or perhaps even a sub-domain.
There is no harm in creating a webshop on an entirely different URL, and directing visitors who come to your original site to that URL.