Hello Raymond,
You may use the following example to achieve this:
Main DID -> floating extention -> AA
Then in night service add this floating extension which should point to another AA or phone number which should be used for the case. You need the floating extension because you cannot select the AA number in the night service mode window. Add also one of the other real phones for night service bell - this is needed because you must use this phone to force night service mode on/off. Then add a code under night service e.g. *222.
This way when user on the phone selected for night service may switch to another AA or external number.
HTH,
Alex