Sure. A CMS doesn't have to be complicated.
Not knowing a single thing about your website, if you had a news section, simply having a server-side script read the news from a database into a HTML page, and another page that would allow you to add to the news database... that would qualify as a CMS.
But if you are talking about using one of them big CMSs, like Joomla, then I wouldn't consider incorporating it into your site, but rather try to incorporate your site into it.
Subtle difference, but it makes a difference ;-)