Hi,
You can edit /usr/share/ebox/stubs/mail/main.cf.mas and /usr/share/ebox/stubs/mail/master.cf.mas.
Those are the templates that are used to generate the postfix configuration.
The only thing that you should take into account is that if you upgrade ebox-mail you will lose these changes. Just keep a copy of those files, and it will be easy to merge back your changes when you upgrade eBox.
Hope this answers your question.