How can I attach hotlinked images in posts/pages within the same server?

No complete code, but I know you can stick it together. 🙂

1. Find all post IDs of unattached images (borrowed from wp-admin/upload.php):

   global $wpdb;
   $lost = $wpdb->get_col( "
       SELECT ID FROM $wpdb->posts
       WHERE post_type="attachment" AND post_parent > '0'
       AND post_parent NOT IN (
           SELECT ID FROM $wpdb->posts
           WHERE post_type NOT IN ( 'attachment', '" . join( "', '", get_post_types( array( 'public' => false ) ) ) . "' )
       )
   " );
   

2. Get image URLs for all post IDs: Since these attachments have no parent their URL is equivalent to get_the_guid() (that’s one of the two reasons the GUID looks like an URL, I don’t like that).

   $urls = array ();
   foreach ( $lost as $id )
       $urls[ $id ] = get_the_guid( $id );
   

3. Now find posts with those images and attach the images to these posts:

   global $wpdb;
   
   foreach ( $urls as $id => $url )
   {
       $posts = get_posts( array ( 's' => $url, 'numberposts' => 1 ) );
   
       if ( ! $posts )
           continue;
   
       $parent_id = $posts[0]->ID;
       $wpdb->query(
           $wpdb->prepare(
               "UPDATE $wpdb->posts SET post_parent = %d WHERE post_type="attachment" AND ID = %d",
               $parent_id,
               $id
           )
       );
   }
   

Not tested, see it just as hints.