we have a mono with 192.168.10.1/24 as LAN , 200.x.x.x as WAN.
Connected to a DSL modem.

Now the ISP is routing a remote branch office subnet 192.168.20.1/24
to the WAN port through the DSL link. The branch office has no
internet access, just a connection to the main office.

We need to allow the 20.1/24 subnet full access to the 10.1/24 subnet
and vice versa.

If we were using PIX then apparently this could be done using
conditional NATting. How do we get the mono to do it?

The solution cannot use NAT as we need the two subnets to be joined as one. 

sai