Apple Id Additional Email

Change your Apple ID - Apple Support

There's special instructions if you want to change your Apple ID to an @icloud.com address.

"If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it." From: https://support.apple.com/HT202667

Apart from the above statement I have yet to find a formal Apple position statement online about re-using email addresses. Many user reports on this forum (I don't have specific reason not to believe them) suggest that contrary to what the statement implies as to ease of address re-use, if you have recently used an email address with Apple for another purpose such as a notification email address it cannot be re-used; at least not immediately and possibly never. I guess you can "try to use it" but many users have claimed this does not work.

About your Apple ID email addresses (was "Manage your Apple ID primary, rescue, alternate, and notification email addresses") --> https://support.apple.com/HT201356

Error message: "You can only add the @icloud.com address that is already on file in your account."

Sept. 2021 Apple Recommended reply --> "It appears you cannot add an iCloud alias as your Apple ID, you can only use the iCloud email address that is the basis for your iCloud mail account which is the old name. If you do not want that as your Apple ID then you need to set up an external email account or you would have to create a new Apple Account." - https://discussions.apple.com/thread/253160200