Tcp/ip Routeing?

We recently hooked up with A Cisco 675 router for a DSL connection. For my PCs 
to
see the AS400 and get on the web I had to assign an ip address, 142.176.xxx.xxx,
to the NIC that is in the same subnet as the router and a host defined on the
AS400. My question is that the IP addresses have run out in the subnet, is there
a way to have the other PC, using the old LAN IP scheme, 196.146.xx.xx, to find
the AS400 on the original host then route back out the ethernet card on the
142..176.xxx.xxx to find the router for internet access. Kind of like a NAT or
something?
Any help appreciated, or am I dreaming again?
Joe Chiasson

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