To add your own fields to the reservation template you need to add this field to the Reservation Template object and add it to the field set.
For every section there is a dedicated Field Set.
You can add your fields to the packaged Field Sets or you can add your own Field Sets.
For the added Field Sets to show up as Sections, they need to either be added on the page builder or the B25__Displayed_Field_Sets__c Field on the Reservation Template Object.
Both need to be a comma separated list.