The host node12 does not appear under any network in the networks list because, as you said, node12 has not been connected yet to any network.
The error you are getting is because the network ‘DMZ_node11' is required on all hosts in the cluster to which you are trying to add node12.
You can either make network ‘DMZ_node11' not required temporarily or connect it to node12.
To make a network non-required go to Compute -> Clusters -> your cluster -> Logical Networks -> Manage Networks and uncheck the required check-box.
To connect a network to a host (e.g. node12) , go to Compute -> Hosts -> node12 -> Network Interfaces -> Setup Host Networks
and attach the required network to the desired interface.