I set up this computer with a Local account.
I have linked it to my 'primary' Microsoft account - which is used in the Mail and Store apps. There are no other accounts linked at this time.
I wish to use the Messaging app with a different Microsoft account. However,
Add an account only lets me add a Facebook account! I don't want to remove or change my primary Microsoft account, since I do not wish to use the Messaging account with any other services.