It would be a touch more involved than adding a custom field – you would need to look at how the PDF is generated and attached and potentially unhook that and hook up your own implementation in its place (which might essentially be a slightly modified version of the original code).
Unfortunately it’s beyond the scope of support to guide you through this one – do know however that the ticketing system is going to change in WooTickets 2.0 and will potentially be easier to amend.