In Outlook, there is no need to create a new ID if you have a Hotmail, Live or MSN ID since they work on Outlook. But if you want to create an Outlook alias, click on 'gear' on top right, go to 'More mail settings'. Under 'Managing your account', go to 'Your email accounts'. You will see an option 'Create an Outlook alias'.
A new folder for the alias ID will be created. Since the alias ID resides within the primary one, you can't login with it. You can send email from an alias ID, 10 of which can be created per year.
Creating one in Yahoo Mail, involves an additional step. Click on 'gear' on top right, go to 'Mail options', click on 'Disposable addresses'. Create a 'base name', which is common to all disposable addresses, which are created by adding keywords. Like in Outlook, a folder for the disposable address can be created. Keywords and base name can be deleted easily.