Customizing is possible in the source code (which is what @Darkland is referring to when talking about an open source project) and you can then just compile your own server version, but I think it would be easier if this is exposed in one of the available templates and I’m not sure if this is the case currently.
You can check on your own, there’s a templates directory in the Mattermost installation directory (by default that’s /opt/mattermost).
I mean you can configure and change everything in the free edition, as long as you know how to code in the languages used by this project and as long as you know how to compile the project from source.
See this documentation for setting up your development environment (which also talks about how to compile the project from source):