Hello,
basically, you would need to route specific traffic to your existing PIX firewalls, and the rest to the new connection, by means of a default route. Let's say your internal network that you need to reach is 192.168.0.0/16, you could configure a static route for that network pointing to your existing PIX:
ip route 192.168.0.0 0.0.255.255 10.1.1.1
where 10.1.1.1 is the IP address of the PIX.
The default route:
ip route 0.0.0.0 0.0.0.0 172.16.1.1
where 172.16.1.1 is the IP address of the new connection, this would take care of the remainder of the traffic (Internet traffic).
Does that make sense ?
Regards,
GNT