If you would like to add an additional email account using the Mail app for Windows, then simply:
- On Start, tap or click Mail.
- Swipe in from the right edge of the screen to open the charms
- Tap Settings.
- Tap Accounts.
- Tap Add an account, choose the type of account you want to add, and then follow the on-screen instructions.
For more information, please visit: http://windows.microsoft.com/en-us/windows-8/add-accounts