That is optional. If you have the possibility to add custom code to your registrar module (if you already use one!!), then you can add the code to the existing function of your registrar module. But then you have to deactivate the domaintool function in the module, because these tasks are then done by the registrar module.
As an example:
Code:
function yourregistrar_RegisterDomain($params) {
...registrar code, which already exists...
Add code from function_RegisterDomain.txt here and edit URL, user and password
}
In this case EVERY registered domain is added in ISPC, not only the one they got with the hostingpackage!
Edit: Okay, I see. I will improve the manual in the next version concerning this.