I have done this by, adding a second IP to the current NIC under the advance tab. Building a superscope in DHCP and include the new IP information. You may need to forward DHCP broadcast to this segment. depending on your switching you may need to add a second nic instead. Make sure you have re-apply the service pack after installing DHCP or SuperScopes will not show up as a option, I believe that SuperScopes didn't exist prior to SP3.