[Serusers] forking with SER

Hi all, I have a question about routing a call to GW when user isn't in location table. If I have a user with SIP address: sip:mister@no.com, and his alias in form of his business phone on office PBX: alias = 1234 can SER use number in aliases and rewrite sip:mister@no.com to <mailto:1234@no.com> 1234(a)no.com? sip:mister@no.com -> "404 Not Found" -> use aliases -> sip:1234@no.com -> t_relay -> GW -> PBX or something like that. So if someone could explain me how can I do this. Do I need some external application to do that and return PSTN number to SER? Or how can I do forking with SER? thanks