If you want a server to act as a slave for a particular zone without configuring a server to act as a master for that zone (presumably because there already is a master server for the zone), select the server you wish to put the slave zone on and then select Add Slave Zone from the Server menu. This will bring up the Add Slave Zone window where you can enter the name of the zone, and record the IP addresses of one or more master servers for the zone. When you are finished click on the Add button.