Hi Pedro,
I assume when you are talking about Call Forwarding you are talking about using the Call Forward All softkey on the phone and not via the UC320W Call forward no answer provisioning...
For the Shared FXO there isn't a way a user can CFA that line in the system. A possible workaround would be to setup an extra phone and make the Shared FXO CFNA to that phones' private extension. You could then CFA that private extension to where you want to route the call.
In the upcoming 2.1 release we will have a (system) night call forward option. When in Night mode you could have a different call forward no answer target.
Hope this helps.
Chris