Why do you have to forward it?
I got in the habit years ago of programming the logic for my ACD groups daytime flow to say "IF #of logged in users in ACD group X < 1 THEN goto alternate number - otherwise continue down through the normal ACD flow logic for that group"
In my case, except for one which is the main company switchboard, I have the alternate number be the same destination I send the group to at night, which is usually our in-house answering service. What this does is if staff arrives late, or they need to have a 15 minute meeting, or it is a holiday and no one is there, or there is severe weather and that site is closed the calls will continue to go to the same place they go at night, and some poor customer is not sitting in a queue until they hang up or it eventually times out and overflows to someplace else.
That's the most convenient way for me to "forward" an ACD number - just have everyone in the group log out. Otherwise there is overflow logic to handle most of the rest of it.