I did this yesterday in-store. Told them I wanted to purchase a PIN to use on the MotoMaker site to get the Moto X. They have not done this very often, so it did take a while. Had to search for the SKU on the computer. It turned out to be "MOTXT1096MM32". Transferred upgrade from Unlimited Data Plan line to a Tiered Data Plan line and then paid for the phone + upgrade fee + tax. They had to go do a few more things on the computer and then an email from Motorola was sent to me that had the PIN in it.
Went home and went to MotoMaker and entered the PIN to customize and order the phone.