Authentication delegation

In the operational environment there is a RADIUS server that is already configured to handle authentication and has all the corresponding accounts. I would like to make use of the Authorization part of RADIUS and install my own server for this purpose. However I need to configure it to pass the authentication messages to the "main" RADIUS server. Is there a way to do this? A secondary radius server configured to pass authentication onto another server? The authorization rules are defined in the secondary server. Thanks for any help.....