You can use both night service and out of service to achieve what you are after.
You will need to setup two buttons
SetHuntGroupOutOfService and the group number (This will only put that one group into out of service mode and will also allow the button to revert the group back to normal service. Using only a button with no group number next it will put all group that the user with button is in into out of service and will not return it to normal service if pressed again, you will need a ClearHuntGroupOutOfService button if you wish to do it this way)
Make up another group call Sales_Divert and put a dummy user in it which has no voicemail, unconditional forward to your outside message company and forward hunt group calls on. The Sales_Divert group needs its ring type to something other than Group.
In the Sales group set the out of service fallback to the Sales_Divert group.
Have nothing in the Night service fallback. If there is nothing there it will send to voicemail or give busy if voicemail is not on the group.
Make a button SetHuntGroupNightService and group number and this will be your voicemail button.
cheers