Q: adding icloud email address to Apple ID

I have an icloud email address I can send to and receive from using my Mail app on my Mac, however, I can't add it to my Apple ID. I get an error message that the email address is related to another Apple ID, but I registered it under mine using the Mail app. How can I get icloud to recognize the the Apple ID that registered it via the Mail app is the same Apple ID on icloud. I only have one Apple ID (with multiple alias emails).