Can I create a custom field or custom button straight from the Canvas builder?
No. At this point, you cannot add custom fields or custom buttons right from the Canvas Builder. You can only add fields/buttons that already exist in the module to the Canvas for better data organization.
If you want a new field/button, you need to go to the module builder, add the field/button and define its properties and make it available for the module first. Then, you can come back and add the newly created field /button to the Canvas wherever required and enhance the way it is presented visually.