Hello, and a (hopefully) simple question

Hey folks. Right now, we use freeradius to authenticate simple pap/chap PPP clients. When a username/password is rejected, radius simply send back a reject message to the NAS. Is it possible to change this behavior so that a failed auth attempt gets accepted with an alternate IP pool instead of being rejected? the idea is to force suspended users through a web proxy that tells them that they have a billing issue, instead of rejecting their connection altogether. Any help would be appreciated.... Vlad