Hi,

Along with xpath the new field to the template try the below code also

class ResConfigSettings(models.TransientModel):
   _inherit = 'res.config.settings'

   def change_report_template(self):
       res = super(ResConfigSettings, self).change_report_template()
       template = self.env.ref(module_name.custom_template_id') #This replace original template by the custom template which you created. 
       return res

In the above function you can pass your template which the new field added.

Hope it helps