What do you mean add to the data? do you mean add the field field_story_image_alt to what is in post_content? I so, then use this:
Update wp_posts set post_content=Concat(post_content,(Select field_story_image_alt from forums_drupal_content_field_story_image where forums_drupal_content_field_story_image.nid=wp_posts.ID LIMIT 1))