Change a single node cluster into an HA-Pair requires a Cluster Interconnect network which is not present in a single node cluster, so you will have to set that up.
For your reference: these are the commands used.
cluster show
net port show
net int show -fields home-port
net int modify -vserver svm_blue -lif nfs_data -home-port e0c
net int revert *
broadcast-domain show
broadcast-domain remove-ports -broadcast-domain Default -ports cl2_n1:e0a,cl2_n1:e0b
broadcast-domain add-ports -ipspace Cluster -broadcast-domain Cluster -ports cl2_n1:e0a,cl2_n1:e0b
net int create -vserver Cluster -lif 1 -service-policy default-cluster -home-port e0a -address 169.254.10.1 -netmask 255.255.0.0
net int create -vserver Cluster -lif 2 -service-policy default-cluster -home-port e0b -address 169.254.10.2 -netmask 255.255.0.0
