Hi and welcome to NethServer Community,
here is the documentation about custom templates.
In the case of /etc/postfix/main.cf you need to create the custom template directory /etc/e-smith/templates-custom/etc/postfix/main.cf first.
Add a fragment file like 90myconfig containing your configuration to the created directory.
Expand the template:
expand-template /etc/postfix/main.cf
Now main.cf should contain your additional configuration.