Add an alias using an existing outlook.com address

Hi - I want to add my new outlook.com address to my primary account which is an sbcglobal account, but every time I tried to add it, I get the message "This email address is already taken. Please try another." Yes, I know it exists, I created it . .  I want to merge it as they tell me I can do.  How do I do this?

Thanks-Sally