You can specify a specific Node and Port with the node command.
http://devcentral.f5.com/Wiki/default.aspx/iRules.node
I don't think that is what you are trying to do though. Are you trying to work around default Virtual Server Address and Port Translations?
Address Translation
Specifies, when checked (enabled), that the system translates the address of the virtual server. When cleared (disabled), specifies that the system uses the address without translation. This option is useful when the system is load balancing devices that have the same IP address. The default is enabled.
Port Translation
Specifies, when checked (enabled), that the system translates the port of the virtual server. When cleared (disabled), specifies that the system uses the port without translation. Turning off port translation for a virtual server is useful if you want to use the virtual server to load balance connections to any service. The default is enabled.
You can de-select these options on the Virtual Server and see if that gives you the behavior that you are looking for.