All transfers between the Converter Agent/Converter Standalone Server to the vCenter server are encrypted. In fact, when you convert a physical machine to a vSphere environment, the connection is made directly to the destination ESXi, vCenter does not participate on this - and is also encripted (being sent to ESXi 443 and 902 ports - encrypted with SSL).
You can check this on KB http://kb.vmware.com/kb/1012382
| Converter 5.x | 443, 902 | TCP | Powered-on Source Windows machine | ESXi/ESX Host | Required for data transfer to destination ESXi/ESX host |
Hope this helps.
Marcelo