Are you asking how to set up Slave DNS on DirectAdmin?
If so search these forums for the Multi-Server Option; that works if you also have a DirectAdmin license on the machine doing the slave DNS.
Or search these forums for Master2Slave DNS Replicator you can use that even if the slave machine isn't running DirectAdmin, but it's my (open source and free) product, not supported directly by DirectAdmin, and it does require setup on both Master and Slave nameserver machines.
Jeff