NAT using multiple subnets

At work we have 3 sites and 3 subnets connected across 3 WAN L3 switches. I would like to change the WAN configuration so that all the WAN L3 switches are connected on a single subnet and each of the other subnets is used just for its sites LAN. However I can not do this at the moment due to NAT used on the firewall on two of these subnets.  Putting all NAT devices on one subnet is not currently an option. I'm not sure if this is possible but can I NAT subnets without them actually being pysically connected to the firewall. So one subnet for example 172.16.2.0 /29 connected to the firewall and a WAN L3 switch, with 172.16.8.0 /23 and 172.16.10.0 /23 using NAT under the same interface on the firewall. We are using a juniper netscreen firewall if that helps.

Current setup is as follows
so for example
WAN switch 1
172.16.8.0 /23 used in NAT 194.*.*.*.* connected to firewall
172.16.10.0 /23 used in NAT 194.*.*.*.* connected to firewall
172.16.12.0 /23
WAN switch 2
172.16.10.0 /23
172.16.12.0 /23
WAN switch 3
172.16.12.0 /23