Freeradius with Caller-station-id

Hi we have several FreeRadius servers talking in our case to SM SQL and these work fine for all our users logging in using Username & passwords. We are just getting a new L2TP link in and with sessions on this link I want to auhenticate *just* on the calling station ID - there will be a username/password BUT these are not what we need. I therefore want to be able to keep username/password for the current sessions, BUT if these fail authenticate on callingtation ID and return IP address details, etc. is there a way to do this ? I worked round it previously by hacking the code BUT I'm sure there is a better way here... Richard