An ISE admin can view, add, and delete endpoints directly from the Context Visibility > Endpoints page. There is no way, however, for an admin to add a new endpoint (by MAC Address) and mark it as 'BYODRegistered equals True' and/or associate that endpoint with a user so that user can then manage that endpoint themselves via the My Devices Portal. Those actions can only happen as part of the BYOD onboarding process.