You can override the podTemplate section of the Elasticsearch spec with pod-related constraints (eg. privilege).
We are looking for running some of ES data nodes on host network (Pod needs to have host IP )so that we can join them to a running ES cluster to migrate ES VM to VM on GKE.
This is likely not going to work. ECK needs to manage the full cluster, it cannot run a subset of data nodes of another existing cluster. Have you considered cross-cluster replication or restoring from snapshot?