Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello
You cannot add an "existing" outlook.com address to another Microsoft account. Any existing outlook.com is already its own Microsoft account, and since Microsoft doesn't support merging Microsoft accounts, this approach is not supported.
Instead, you need to do it this way:
Can you sign into your personal account via this link: https://account.live.com?
If so, go to Your Info > Sign in preference, Add an alias.
Create new: this will allow you to use the native domain outlook.com to create a new alias.
Once added, make this new alias your primary alias.