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