Short answer, no.
Longer answer …Mail-in-a-Box was created to be an easy solution to creating a personal mail server so that people could get away from the data mining by the big guys … this project is not overly flexible so making the type of adjustments you want to make are (NOT) feasible, they are not resonable for this project.
And then one would have to ask the question – why??? If there is some reason that it matters, then the solution is to spin up another instance of MiaB.
There is no reason that the box cannot host additional domains … mail servers usually host many domains. Only domains hosting name servers require glue records …
But the IP address cannot be used for more than one mail server hostname as that is a security check that most email recipient servers check (rDNS). You can only have one rDNS per IP address. So the answer is NO, it cannot be done.