Yes the rDNS record doesn't need to match the actual domain in this case. It only needs to resolve to A domain (which should resolve back to that ip as well). On a DA server this usually is somethine like server.hostingcompany.tld - but it can be whatever domain you want.
Most datacenters/isps handle the PTR records for IP addresses. Theres a big chance adding them in DirectAdmin is not going to work.
For your IP address its ns1.nac.net which answers the PTR request.
edit: the correct term for a PTR record to point to is a FQDN, which consist of 3 parts of a domain (server.domain.tld). Im not sure, but it might be a good idea to make it the same as the FQDN in DirectAdmin, which is also being used in mail.