Unbound not really meant to be authoritative - but you can for sure answer with authoritative responses, ie SOA and create pretty much any record you want. But your not going to be able to create those records in the gui. Why do you think you need a full blown authoritative NS? What exactly are you trying to do - if all you want is to respond for some MX records... Just do that in the custom option box

After a bunch of googling: unbound is not ever authoritative dnsmasq CAN be authoritative. I'm working on it...