Generating AAA message for freeradius.

Hi, I am working on Authentication module in NSIS protocol suite ( http://user.informatik.uni-goettingen.de/~nsis/). The scenario is as follows: User A-------- NSIS server---------Radius server User A sends the NSIS request with its keyed hash (generated using User A's key) appended to it to the NSIS server. Now the NSIS server needs to authenticate that request with a Radius server. Is there a way to do this i.e. how to generate a AAA message with the information available i.e. a string, its keyed hash and User A's id ? Assuming that the Radius server has keys of all legitimate users. Regards, Prateek