Try to update the function message_post of a related model. An example for project.issue:

@api.multi

    @api.returns('mail.message', lambda value: value.id)

    def message_post(self, subtype=None, **kwargs):

 self.ensure_one()

 mail_message = super(ProjectIssue, self).message_post(subtype=subtype, **kwargs)

 if kwargs.get('body'):

  if self.description:

   description = self.description + kwargs.get('body')

  else:

   description = kwargs.get('body')

  self.sudo().write({'description': description)

 return mail_message

This code would add all the messages bodies to description. You may add other conditions to restrict messages, e.g by subtype or when authors are portal

P.S. for newly created issues update the function message_new