Ability to add a custom document to an email scenario

It would be really convenient to be able to add a custom document attachment to an event save email scenario. Example: when student orgs rent a vehicle we send them a reservation confirmation and they are required to fill out a pre-departure trip form. I built a confirmation scenario/email and the form as a custom document, but find I can't link the two. (Can I?) Therefor we'll need to continue sending confirmation emails manually if we want to send the trip form in the same message.