Not sure if this is possible. However as long as the generated email is sent to an imported mailbox, it will update the ticket. You do have to have your WHD set to import messages.
E-Mails from WHD will create new E-Mail which go back to WHD. You could add a link in the E-Mail Template to encourage your users to login via the Client UI with that address
Hello, this isn't possible however I have listed the Subject-specific commands you can use here to interact with the help desk:
Action: Tech update - Adding notes
Hidden: Yes or No for client visibility
Status: Open, close, pending etc.
Email Client: if you want the clients to receive the email
Reassign: Reassign to another tech
Minutes worked: place minutes worked
Billing rate: make changes to the billing rate.
On top of this, you can use the items below as a guide:
Action:Cancel - client ticket cancellation
Action:Update - client add note
Action:ConfirmResolution - client confirmation
Action:RejectResolution - client confirmation
Action:ApproveTicket - tech approval process
Action:DenyTicket - tech approval process
Action:TechUpdate - tech update
Action:TechDelete - tech delete ticket
(for those who know these already, if you know some I've missed off the list then please let me know)