Mail-in-a-Box is a project designed to make it easy to deploy a single server turn-key email server. It’s tools do not explicitly support non-standard configurations like this and there would be no guarantee that things would not seriously break. I would strongly recommend against using Mail-In-A-Box to get started with this type of configuration. Instead you should look at the components of it (Postfix/Dovecot) and work from those to set up and manage your configuration directly.