Dang that sucks your expedition didn't come with a keypad, I always thought it was standard. They are definitely awesome to have.
I copied this from another site:
http://www.fordaccessoriesstore.com/fas/b2c/catalog.asp
The Ford Genuine Accessories website now has keyless entry keypads for sale--and for just $85. No drilling... it just sticks on and emits the same frequency as your remote key fob. I
The description shows how the thing operates:
Conveniently unlock your vehicle without your key or keyless entry remote. No wiring necessary – programmable, backlit keypad tunes to the same frequency as your transmitter. No-drill installation. Comes with two covers: one is primed (paint to match); the other has a black textured finish. For vehicles equipped with factory remote keyless entry.
The website doesn't have one specifically for an expedition, but maybe the one for the fusion(07-10) will work since you just program it to your transmitter. Did you try calling the dealership and asking about this?