If I understood you correctly, you need to change the contents of the Guest Voucher, which a web admin can create.
Bad news: WYSIWYG. You cannot change/add/remove anything that is not listed in the web-admin GUI. I think that Moto is just providing some features like CP and vouchers "included in the box", which are great when you don't need much.. There are much better customized CP solutions out there for sure, but they will cost you extra.