No, they are not. You need to create a separate user account for AGOL. You can think of AGOL as a separate, really big Portal instance with its own logins.
Now, a single AGOL login can be transferred between organizations on AGOL, since the underlying authorization system is the same. But Portal is totally separate.
and you have options for single sign-on / active directory to both portal and agol that might meet the needs of your request