Why I am not able to add another microsoft account in add alias option?

I am trying to add another microsoft account in the add alias option in Accounts settings where it asks for an existing account. On providing a hotmail account it says "This email id is already taken. Please try another one.", of course it is taken, that is why it is an existing account.

Please help me in adding another Microsoft account so that I don't waste time switching to accounts or using two different browsers at a time. :(

Prince