Do I understand you correctly? The Cluster currently has 21 nodes, all using local ZFS for storage?
Adding more Nodes which do have a Ceph cluster should work. If the other 21 nodes should also access the Ceph storage, make sure they have a (fast) network connection to the Ceph Public network.
If you only want those 3 Nodes to access the Ceph network, you can limit it in the Datacenter -> Storage -> <your ceph storage> configuration.
If you limit the storages on specific nodes and use HA, make sure to use HA Groups and restrict VMs to only the nodes where they can actually access their disks.