Join 2 networks and DNS entries

I got a company network with domain names * and a subnet of This network restricts access to Internet via proxy. Let's call this NET_A
There is a different network that it does dhcp and no restriction to internet.Lets call this NET_B
i would like to "merge" those 2 networks and have a new subnet for a small portion of devices. this network will be (NET_C) and will route everything to the unrestircted network (NET_B), unless there is a request to URL(s) * which will be router to NET_A

The devices in NET_C should not have proxy settings

My thought is to have a PC running OperWRT and have 3 ethernet cards:
-eth0: NET_A
-eth1: NET_B. Default gateway will be here.
-eth2: NET_C

Up to now i got working only the NAT to eth1 (NET_B), unrestricted access to Internet. I do not know how to route to eth0: NET_A when there is a call to *