Hi Jay,
You don't need to use another interface to configure this. All you need is the following:
1) Your ISP to route the entire new IP range to the current outside IP address of your firewall
2) Create static transltations to the new range specifying the current outside interface in the static command.
eg:
Current outside ip = y.y.y.y/24
Your ISP routes x.x.x.x/24 towards y.y.y.y
static (DMZ,outside) x.x.x.x d.d.d.d netmask 255.255.255.255
This solution is used often!
Good luck!