You don't add a host, you add a cluster of hosts. We basically detect all hosts in a cluster, and all should be prepared to allow load balancing across all hosts.
When you add a Provider and need to prepare the cluster, vCloud will probably try to prepare all the hosts at the same time. This is normal.
However, you can cancel the Maintenance Mode request from vCenter to stop the process on all but 1-2 hosts in the cluster. Once they are prepared, you can then prepare a subset of hosts from the appropriate screen in vCloud Director.