Inviting Existing Contacts of Existing Supplier isupplier

We are implementing iSupplier and want to invite existing contacts of existing suppliers to register on isupplier.

I see there is a way to invite a new contact for an existing supplier but I don't see an LOV where I can select existing contact for existing supplier and send notification.

Also I see an API available to bulk  register contacts of Supplier (metalink note 256790.1). Can someone give me idea it works for new contacts only or does it work for existing contacts as well.

Thanks in advance.