What you are looking for is called IP address aliasing. In Mac OSX you can create an aliased address by issuing the following command (see the ifconfig manpage for more details):
sudo ifconfig en0 alias $ADDRESS $NETMASK
However, you cannot receive DHCP leases on these "virtual NICs" (see here for why) so you will need to have an available static IP address and know the appropriate gateway, all information you can get from your ISP. Depending on what your ISP's upstream subnetting and routing looks like you may also have to add a static route (see the route manapage).
