You can set up a guest mailbox and enable outbound transfer, but that still requires another button press from the user. i.e.: the user will call in, get the AA, enter the "extension", hear the mailbox greeting "press 7 to be transferred to my cell", outbound transfer.
If you want to just do a straight transfer, you need to have an active station to handle it. If you have an ASM in the system, you can use one of those ports rather than tying up a physical phone. Digital ports need a phone attached, otherwise they go inactive and won't forward.
I suppose you could also do something fancy with the routing table (code = extension, absorb all, route to the line pool and outdial the cell #), but that's getting a little hackish.