Hello.
I do need to one time migrate 3 servers from the ESX 4 cluster to the ESX 5 server. Can i add the ESX 5 server to the exisitng vcenter, switch off the VM and migrate?
You will need vCenter Server 5 to manage the ESXi 5 host. Once you get vCenter Server to v5, then yes you can power off the VM and migrate it to your new cluster.
Good Luck!