Can you please provide some additional context for exactly what you want to do because I believe that there may be some misunderstanding about what IP Spoofing is and how it works with LoadRunner\Performance Center.
First, IP Spoofing (IPS) is only to be used on Hosts configured to be Load Generators and Stand Alone Load Generators (LG). The purpose of IPS is to provide each LG with several IP addresses that can be used to replace the real IP address in certain protocols communication streams.
You do not install IP Spoofing on your Application Under Test (AUT) Server, you do not install it on any other type of computer or component (for example DB\Application servers or switches, load balancers, or firewalls). Only install IPS on an LG.
Second, IPS has several requirements that make using it on some random load generator out on the Internet difficult. Here is a partial lists of concerns.
a) IPS requires that the LG be setup for static IP Address. A computer setup to use dynamically assigned IP addresses will not work. IPS also requires that there be available IP addresses in the rage of addresses that the LG is using.
b) IPS permanently assigns the newly generated IP addresses to the LG. When you look at the networking settings after the IP wizard is run you will see that all the IP addresses are listed. This means those IP addresses can not be used by any other computer. This is difficult to control for a random computer on the internet. Most ISP's do not provide a static IP address and they do not allow for a computer connected to there systems to block out multiple IP addresses.
c) I have never seen IPS work with Over The Firewall setups. This means that unless you have an LG that is out on the internet that doesn't have any firewalls (seems highly unlikely) IPS will not be an option for you
d) IPS works only for a very few select protocols. Web HTTP\HTML is by far the most common protocol that can use IPS. TruClient and any protocol with GUI in its name will not use IPS. Citrix and RDP will also not be able to use IPS.
Finally, IPS is a trick. It is a trick that is used by some hackers to perpetrate Denial of Service attacks. Because of this there are many security software companies out there that have created software at the firewall, switch, and server level that is specifically designed to recognize a spoofed IP and kill communication with all the IP addresses in the rage it determines an attack is coming from. IP Spoofing therefore is designed to be used in a test environment, where you can control what security software is deployed that could prevent the LG from doing it's job. For a vast majority of situations this means that the test lab and the AUT servers are installed as part of an Intranet, and not the Internet.
If your goal is to set up LG's out in the Internet to simulate load from the outside world to a server that is housed in your companies environment this is possible, but I would suggest that IPS will not be possible and you will need to have multiple LG's, not multiple IP Addresses on just one LG.
Hope this helps. If not, please provide a more precise description of exactly what you want to accomplish and the community can provide additional suggestions.