Actually it can be done by using Feature 84. I assume you have "allow redirect" on.
When you press Feature 84 it will ask you to select line out, which normally is intercom.
Then it should ask you to enter the phone number to forward to, you might need to start it with a 9, or what ever you use to call out.
Then it should ask you to select lines to use, which would be the button the DID is assigned to you want to forward.
The display should then say "redirecting"
Then test.
To cancel just press Feature #84 and just hit "ALL" just to be safe. Unless you are redirecting multiple DID's to different numbers. Then you you hit the DID button you want canceled.
hope this helps