It sounds like you should look into masquerading / mangle.
You can do what your wanting a few ways.
iptables -t nat -A POSTROUTING -o <your interface> -j MASQUERADE
the above would take everything behind your nat box and apply it's ip address to it. So, if your nat box had an interface on the same subnet of your destination, it change the SOURCE IP. However, it will on change it to the IP of the interface sitting on the same subnet as the destination / interface.
The other way:
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 192.168.2.1:192.168.2.10
That would take all traffic OUTBOUND on EHT0 and mangle it to be what ever ip range you wish to have. YOur going to need to use a few more rules though. The above example does not cover everything you will need to do. You should look into DNAT and SNAT.
Hope it helps.