Prospective vendor registration through iSupplier portal

Hi Gurus -  Does any one know how  a supplier can register themselves as vendors through iSupplier portal ,If so ,Can anyone suggest how to do this?

As far as I know ,We can invite the supplier users through Purchasing-->Supply Base-->Supplier Registration --> Invite Supplier user

This navigation works only for existing suppliers. We want to do this in such a way that the suppliers can request themselves as prospective vendors.