You typically do this with a Phantom Extension, then Call Forward the phantom extension to the external number using Remote Station Programming.
Build a new Phantom Extension, then use Remote Programming FAC from a working phone...
359 (Remote Programming FAC by default)
xxxx (extension of Phantom you created)
yyyy (Password of the extension, by default the same as the extension number)
355 (Call Forward Always FAC)
8 (External Access)
npanxxxxxx (Destination number as you would dial it from a phone)
SPEAKER
The display should say CALL FWD ALWAYS TO xxxxxxxx (the number programmed in).
Then set the Ring In Destination for the number in question to the Phantom Extension.
There are some caveats to this, and probably a way to do it with DEE, but this is the way I always do it.