I can't really tell if it's possible to start with a plain Django project and then add Mezzanine. I have a project or two that could use a better CMS, but only for a very small subsection of the pages.

The documentation seems to indicate that you really need to start with a Mezzanine project, and then add all your Django bits. It makes a lot of sense for a huge number of sites, but perhaps not so much for the type of projects that are primarily need an easy editor for just a few pages of the overall site.