Where do you want to add this info (Which Field do you want to be updated)?
The *ref# email update will only add an Incident Detail record by default (EMAILIN). In order to have the email update a specific field, for example Work Notes, you would need to create a business rule for the Mail Listen module.
It is certainly possible but not easy.(At least in our old SDE version 10.1) For instance, we created a new field on the incident form for Firewall Request approval using one of the fields in the database that is not used (HelpDeskText06 as an example). When a user fills out a firewall request, an email is sent to security via a business rule. Security clicks on the approve or deny button in the email and another business rule populates the field in the form with the Approve or Deny string. Business rules can do anything you like but it takes time.