If you have a gateway email placement and want to cluster them, it will cluster web messenger emails, users, keys, managed domains, and policies.
If you have an old server and are replacing it with a brand new cluster, it would be easier to just export policies and reimport to the first online UN, then create clusters off that one. If you have all the other things too, like keys and web messenger, you will find it a lot less hassle to create a new member, cluster the data, and promote it to the parent(i think they call them sponsors in PGP), then you can remove the old server.