ok, now I read it!
I think it should be possible to do the same from the Server Manager. The example from virtualmin.com could be implemented with Users > Edit > Services > Forward messages > Email address. It changes the /etc/postfix/aliases right column, so no need for templates-custom.
However - let me say - it’s a mess! POP, forwards, multiple relay domains, MX records… I would not like to manage similar environments! 
As @alefattorini said above, we lack multi-office support. To have it, we need to modify a lot of stuff. Assuming a single PDC domain on different offices the list could be:
getent()-based Users/Groups module- multi-branch LDAP DIT layout
- mail server tables from LDAP
- …
I’d like to work on it, but also NethServer 7 is on the roadmap…
But I don’t want think about it now: I’m going on vacation, next week 