Do you mean you want to create a secondary account using the @comcast.net domain? Or do you mean you want to add an external account you have like Yahoo or Outlook.com to the folder panel at the left of the Inbox page? If the former,-----------------------
You can have up to 6 secondary accounts in addition to your Primary. Here are the instructions-------
If you mean an external account, click on "Add mail account" in the left folder pane and follow the prompts.