I don't believe so. A firewall is essentially a router, so that means the IP address space on the "outside" must be different than the IP address space on the "inside". Recently a friend setup an ISA server (before eventually buying a Cisco PIX firewall) and what I told him to do (which worked) was to:
A) give each PC on the "inside" an IP address. (i.e. choose an IP addressing scheme for the inside, like 192.168.x.y or 10.x.y.z). Don't use DHCP for this, as with a business class conenction with multiple IPs, you should get 16 static addresses, not DHCP addresses. If your provider is forcing your to use DHCP for 16 addresses on a business-class connection, then complain and demand static IPs. Otherwise using DNS to direct people to your servers will be a great pain (plus that makes using a firewall virtually impossible.)
then
B) Setup a static NAT entry for each of your 16 "outside" addresses (well, actually 15, because usually the ISA server would need one of them) that points each outside address to the proper inside address.
HTH,
GoldChain