post-proxy

Hi, What i would like to do is send an attribute in the radius answer but only for proxied request. When the reply comes back from a distant radius server, I would like to add something like : Filter-ID = "Enterasys:version=1:policy=test". But i want to add this only for proxied users which means i cannot use (I suppose) the users file. is there a way to do it? Thank you.