[otrs] Setting ticket keys in follow up

Hi all, I'm currently using the latest version of OTRS 1.3.x and have noticed that X-OTRS-TicketKeyX and X-OTRS-TicketKeyValueX are not being set on follow up e-mails. Are the ticket keys only settable during ticket creation and not on replies? Is there a config option I'm missing? If this is the expected behavior, would it be possible to give us an option in 2.0 to allow ticketkeys to be set on a follow up? Best wishes, Paul