I have use another product that if I was emailing outside the service desk I was able to add ##TicketNumber## to the subjuct line and it would add it to that existing ticket and not create a new one. Does Service Desk have this feature?
Such functionality is not yet available in the Service Desk (SD). As what you are suggesting is somewhat related to your other forum post, this can fall under QOL improvements in SD. That is, when you send an email to SD with a certain ‘keyword’ in the subject line, SD will still create a ticket but it will automatically append (merge) it to an existing ticket.