Yes, the number of nodes in the secondary system has to be equal to the number of active nodes in the primary system -so you can only add standby nodes.
below link takes you to admin guide section "Use System Replication for Near Zero Downtime Upgrades" - this explains how you can use secondary for minimal or zero down time maintenance of primary system:
SAP HANA Administration Guide - SAP Library
hdbnsutil - is the command line tool which helps you to setup and work with system replication.
Set up System Replication with hdbnsutil - SAP HANA Administration Guide - SAP Library
to add host you have to use resident hdblcm or hdblcmgui or hdblcm web
Adding Hosts to an SAP HANA System - SAP HANA Administration Guide - SAP Library