Every single Service Desk competitor is able to strip out the new text from an email reply and just import this, using various techniques. Even the old Helpdesk product did that very effectively, and as far as I weas aware managed to do this without having to put in special markers to help distinguish between new and old.
It is therefore not impossible and is an essential requirement. Added to this the new test should be added to the comment table and not the process event table.