Netscreen 50 Different NAT'ed Public IP for Internal Subnet

We are using a Netscreen 50 and need to have an internal subnet show up as a different public IP than the untrusted IP. The subnet is not on the same subnet as the trust interface but is accessible through the route table. All outbound internet traffic from the entire network goes through the Netscreen and gets NAT'ed using the untrust IP. That is fine, however there is another subnet that needs to have a different NAT'ed IP address for internet communications as the content they will be accessing is restricted by source IP. Can this be accomplished using the Netscreen, and if so how? Thanks, Ben