Make existing icloud email address Apple ID icloud email

I'm using iCloud for Windows, because I want to access my Apple Calendar in Outlook. I logged into iCloud with my Apple ID. I have to create an iCloud email address to use iCloud Mail with Outlook. I have an existing [abc]@icloud.com email address I want to use. However, years ago I set up [abc]@icloud.com as a separate Apple ID. Now I can't choose [abc]@icloud.com because it's already taken. Can Apple customer service attach [abc]@icloud.com to my primary Apple ID? Alternatively, is there a way I can inactivate [abc]@icloud.com as an Apple ID so I can select it?