To Add an additional node to an existing Secure system, you will need to follow the first couple of steps for the Secure Configuration Wizard.
When you reach the New Node Setup screen:
Choose Add a node to an existing Network Secure System (or join an Archive-only installation) from the list.
Enter and verify the existing password, then create a a new one and verify.
From the New Node Configuration screen;
Enter the IP address (or choose Detect).
Enter the Existing Node Admin Password and verify.