I have no experience of the RT-N56U. I know of no reason why you wouldn't be able to use it with pfSense.
If you search the pfSense forums for N56U
you might find a report from a user of the N56U.
In the pfSense documentation collection at http://doc.pfsense.org
there is at least one item on connecting external access points (in the How-tos if I recall correctly).