Using inbound mail to add a note to an active Incident

Is it possible to have Mail Manager add a note to an active incident.  For example, a user emails the helpdesk inbox and an incident is automatically created in Service Desk.  Service Desk then saids an automatic notification to the use with the Incident number.  Users have a tendencay to reply to this email which is then delivered to the helpdesk inbox and a separate incident is then created.  I'd like their reply to be added to the notes section of the incident that has already been created.