If your system goes into "night mode" after hours then yes it is possible.
To do this create a system speed dial that routes the call to the external number. Be sure to tick "over rides toll control".
Next, go to the call rerouting always form and choose a vacant number other than '1'. For example you may be able to use "20".
In each field choose "reroute" and then in the destination field put the system speed dial number you created above.
Next go the the Call Rerouting Assignment form. Find the 6 phone you want to forward and put the call rerouting always number we chose in the step above (Our example was '20") in the Night 1 and Night 2 columns.
All done.
Ralph