Yes you can, we just did last week!!
You can go over to Beach Club and talk to the agent in the lobby there, or go over to Pop Century and purchase something in the Food Court, and your receipt will have a phone # on it.
The current offer expires December 31st.
The phone # to call has been posted on the resorts discount board.
You must be a Disney Resort guest and you must book before you check out on your current stay before December 31st.
You can book one room per adult on your current stay.
A $200 deposit is due at time of booking.