To resize the cluster by adding nodes you simply upgrade the release specifying the new number of nodes you want. Of course, the new value must meet the requirements for elastically expanding the cluster, as set out in the discussion of adding nodes to the cluster in the VoltDB Administrator's Guide. So, for example, to increase the cluster size by two nodes, you can set the replica count to seven:

$ helm upgrade mydb voltdb/voltdb       \
   --reuse-values                       \
   --set cluster.clusterSpec.replicas=7