The only way to have the ban reason included in a forum ban is to ban the user from the control panel. When you ban the user from back end there is an option to leave a note as to why they were banned. This option doesn't carry over to the front end and warning system. I've looked for addons for this and have come up empty. As much as I'd like to see this implemented we're just going to have to wait until Xenforo adds this in an update or until a addon is made.