You cannot add an extension number to the end of the phone number in order to speak with a specific operator.
All Italian numbers consist of 10 digits.
However, you can dial the extension after calling the number.
Do this by entering the extension (e.g., 101) while listening to the switchboard message.
P.S.
Can we give you some advice?
For the system to operate properly, use internal extensions with 3 numbers rather than 2.