I think you're confusing ISA firewall Networks and Network Rules with something else.
ISA firewall Networks are used to define what addresses are spoofed. When a network interface on the ISA firewall receives a packet from a source IP address that is not part of that ISA firewall Network, then it is spoofed.
For Network Rules, those are used to determine whether two ISA firewall Networks are connected, and if they are connected, whether there is a NAT or Route relationship between them.
It is possible, but, in fact, ISA ovewrites Routing Table and Routing And Remoute Access preferenses at every startup. I found nothing about routing in ISA help (exept Network Rules) and I got no answer from isaserver support (http://forums.isaserver.org/m_2002017169/mpage_1/key_/tm.htm#2002017169 I do not need it now, we refuse tunnel connection)
That is the reason for me to think that this feature is missing