Just adding the extnd-call button should do it.
It looks up the extension mapping, and sends the call to the mapped cell phone number.
Alternatively, you might add a DID number to the feature extension pages that will allow you to pick up the active line. Put a speed dial on your phone to call that DID number, and when you call it, you'll be in a 3-way call with your station. You can then hang up the station.
Either way should work to transfer a call from your desk phone to your cell phone.
Carpe dialem! (Seize the line!)