I cannot add my existing outlook accounts as alias to my existing live account which is my primary account?

I a live account which I use as my primary email. Beside live account I have also two outlook accounts.

I have tried to add the outlook accounts as alias to my live account. But I keep getting this message "This email address is already in use. Try another."

It is also not possible to add the outlook accounts as alias to each other, and when I instead try to add my live account as alias to one of my outlook accounts, then it tells me that I cannot add a live account as alias.

It is actually very important for me to add my outlook accounts as alias to my live account, is there any solution?

Thanks in advance