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.
There's a section about integrating into existing projects in the README, which forms the overview section of the docs, as well as the front page of the GitHub repo:
Agree completely. This should be front-and-centre in the documentation. A CMS built on top of Django should, by default, be just another Django reusable app. It shouldn't take over your entire project.
The `mezzanine-project myproject` bootstrapping command should be just an optional shortcut for projects which are completely CMS-based. Which, in my experience, is the minority of projects.
Mezzanine provides a platform that allows you to do a lot more than just CMS stuff, so some projects aren't completely CMS-based. The mezzanine-project command just creates a django project that has some settings already configured (installed_apps, middleware, urls, etc...) so you definitely could integrate Mezzanine into an existing project. In reality Mezzanine is a Django reusable app.
This came up on the mailing list a few weeks ago and it is doable. The Docs are geared towards new projects but you'll want to look at the INSTALLED_APPS and the urls.py to see what you'd need.
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.