NAT and static IP address with conexant router

I have an eTec conexant router with 1 IP address and I am using NAT with
port forwarding quite happily. I now have need to do a few tests for a short
while which needs real internet addresses so I went and got a range from my
ISP. I want to keep my NAT and use the IP's I have for these tests.. ( I
dont have enough IP's for all machines tho) and I dont want to disturb the
current forwarding I have. Are there any guides for this kind of setup? Can
anyone help?

Thanks
Infant Newbie