Adding an attribute to an incoming Access-Request

I have played with this a bit and can't seem to get it working... I need to add the "NAS-Port = 0" attribute to an incoming request if it is not set. This is currently breaking my sqlippool config and the upstream partner making the requests is not giving me a NAS-Port attribute. It looks like this should be done in the preprocess section, however the hints looks like it only works with a username and huntgroups doesn't really do what I need... Anyone got an idea? Thanks, Roy