I am not really sure off your design. But PIX has no problem Natting between any subnets.
Are both of these networks connected into diffrent interfaces in the PIX.
Like Inside, Outside, DMZ1, DMZ2 etc.
The Security setting that you place on the firewall manages whether you need to nat. When going from lower security to higher security etc.
Look at the Global and Nat commands in the PIx. These should help. Use the Static when you are comingfrom outside to inside as you are.
Also, cisco has some great DOC’s on Natting in the PIX.