ICENodes must be defined first before they can be applied in Softimage. The registration process must happen in the XSILoadPlugin callback when the ICENode self-installable plug-in is loaded. ICENodes are defined with the ICENodeDef class and registered with PluginRegistrar::RegisterICENode.
You can get your Custom ICENode to appear in the Preset Manager with a special icon (thumbnail). See Customizing Icons for Custom ICENodes for more information.