Managing personal Outlook.com account settings, security, and privacy
@J W said
When I try to add it back, "Add an existing email address" says it is part of a reserved domain
I am sorry. It is gone and can not be recovered.
This is a huge hole in Microsoft Account Security that many, people have fallen into. I realize that this will be of no help to you, but to perhaps keep someone else from falling into this same hole, I suggest that you Vote for and Comment on this Feature Request.
Remove/Delete Account Alias - Grace period to restore
https://feedbackportal.microsoft.com/feedback/idea/6ab87431-1b9a-f111-9b47-6045bd856709
(You must "Sign In" at the top right of the Feedback page before you can vote).
there a way to contact an agent for this?
No. Microsoft Account Security is an automated process. Microsoft Support Agents can not unlock an account, reset your password, or change any Security Settings no matter what documentation you can present.