This is a good idea, and has come up throughout the years. It has not been built in to the device, as the primary use-case for the RAP-2 is for teleworker or fixed-remote-worker environments.
That being said, I have put together my own 'kit' to run the RAP-2 in such circumstances with the use of an ASUS 802.11 bridge. They are half the size of the RAP-2 and can plug directly into the E0 port... the result? a wireless 'backhaul' enabled RAP-2.
Not ideal (due to cost, and carrying two boxes)...but it does work!
JF