Our ArcGIS Online is federated so, users can login with their organizational USERNAME and PASSWORD. When in Portal for ArcGIS I want to add data from ArcGIS Online, which is not public, it asks for Username and Password, not allowing me going through the option to use our organizational username and password. If I don´t have a named user, but and organizational Username and Password, how can I login to access de data?.
I think that what you are trying to do isn't currently possible if you want to store credentials with enterprise logins. There should be at least one built-in user in your ArcGIS Online. If you have access to these credentials, I would suggest using that to store the credentials (provided it has the right permissions to the items) in Portal for ArcGIS.