As of v4.6.5.4 of NxFilter and NxCloud, we have conditional forwarder feature on 'DNS > Conditional Forwarder'. You can add as many as domains you want there and you can set different DNS servers for the domains you added.
One thing to note is that you have to use a wildcard there as we use a fast lookup method with Conditional Forwarder.
Code:
ex) *.mydomain.local, *.0.0.192.in-addr.arpa