I have seen this request several times on the forum. As others have pointed out, a mail server is a vital piece of infrastructure that should have minimal downtime. The more things you place on a server the more likely you are going to experience downtime.
Having said that, I recently thought of a good way to move CMS onto a MiaB installation. You could modify the Nginx server to run on ports different than 80 and 443. That way you could install Apache to run your CMS and other web infrastructure without disturbing the MiaB installation. I have tried running the Nginx server on different ports and both control panel and Roundcube seem to work fine.
To try this you need to fork the code on GitHub and modify
conf/nginx.conf to change the ports and then install MiaB from your repository. If you don’t fork the repository, then any updates to MiaB will wipe out your configuration changes in
/etc/nginx/conf.d/local.conf. In order to get any updates to MiaB you will need to then merge them into your repository when they become available.
Also you will need to configure Apache to allow for the Letsencrypt authentication.