Is it possible to get packet id in pre-proxy section?

Helo guys, One side auth with pap method, but the other side needs chap auth method, so I have to do some translating work. chap-string = Packet-Id + Cleartext-Password + authenticator chap-password = packet-id + md5(chap-string) Is it possible to get packet id in pre-proxy section? how can i pack packet-id & authenticator into chap-string? Any ideas welcomed. many thx