NAT over multiple subnets

[lycanthrope]

hello
I set up NAT+ipfw on FreeBSD computer which resides on 192.168.2.x LAN, and has an outgoing ADSL connection. any client on 2.x can access internet by setting this freebsd box as its gateway. there is another server on the 2.x network which is used as a link to another LAN, 1.x subnet. those users can not use NAT on the freebsd box,since their gateway is a 1.x server,connected to 2.x server. how to enable users from 1.x network to use NAT on freebsd machine,without altering 1.x or 2.x servers? note that 1.x clients know how to get to the freebsd machine (quagga-ripd), but can not alter their gateways to the freebsd machine,since it has to be the 1.x server.
so, how to enable 1.x user to use NAT on freebsd router?

sorry for a rather long post, and maybe slight confusion,but I believe you know what I have, and what I would like to do.

thank you for all your help!

ps: NAT is not mandatory,some other mean of internet sharing is acceptable,as long as it is stable and fast on BSD