Managing personal Outlook.com account settings, security, and privacy
Hi,
As I understand you want to accounts to share one mailbox. Let me assist you on this.
If you have two Outlook.com accounts, you can’t add this as an alias for the other since they are already Microsoft account.
If your third party account was already registered as Microsoft account, you cannot also add this as alias. You can only add an existing email address as an alias if it is not yet registered as Microsoft account.
If you want to manage several accounts in one mailbox, you can use mail client like Windows Live Mail. You can also use Outlook.com to send and receive emails from other account that uses POP. You may check this link for more details and how to do this: Send and receive email from other accounts in Outlook.com
I hope this helps. Should you have further questions, please let us know.
Thank you.