[Dovecot] appending domain based on port

I am looking for a way to append a domain to unqualified user names based on the port that the client connected to. Like this:

port 9110 - @test.com port 9111 - @other.com port 9112 - @third.com

it looks like auth-default-realm does this, but you can only specify it once for the whole process.

I know I could do this by running a separate copy of dovecot attached to each port in question, but I would like to do it all in one process if possible.

Other questions I have, if I use the auth-default-realm:

1. does it happen before auth lookup I assume?
2. do I have to define the domain i use here as an auth-realm in addition to default?