Thank you for posting your question here in the Xfinity Forum. Unfortunately a username and/or email can only be tied to one account. You can use the email on another account as a secondary contact email, but you would not be able to manage it when accessing the other account.
Also, we now allow users to access their comcast.net emails even if they no longer have a Comcast account. In order to keep that email active the user must log in within 90 days of the account closing and then at least every 90 days after that.
If you have a email and/or username from a previous account we can move it over to the new account. If this is something you are interested in, please send me a private message with the username/email you want moved and the following information for both accounts.
You are welcome!
Feel free to author another Xfinity Forum post if you have any other questions.