If I'm understanding you correctly, you want two separate nets, but with a wide open bridge between them. I guess the question is, why? If you just want to use the SME server's applications, put it in single user mode and have the whole thing on a single net.
However, if you truly want to have separate nets, but full two-way access, SME is not going to do it for you OOTB. You would have to redo (ie, eliminate) the integrated firewall. Not a simple task. You can, however, purchase a LinkSys or D-Link two-port router for less than $100 that will create an open bridge for you. That might be the easier thing to do.
Scott