function ikinci_sku() {
  $args = array(
    'label' => __( 'İkinci SKU', 'woocommerce' ),
    'placeholder' => __( 'SKU giriniz', 'woocommerce' ),
    'id' => 'gizli_sku',
  );
  woocommerce_wp_text_input( $args );
}
add_action( 'woocommerce_product_options_sku', 'ikinci_sku' );

function meta_kaydet( $post_id ) {
  $sku = isset( $_POST[ 'gizli_sku' ] ) ? sanitize_text_field( $_POST[ 'gizli_sku' ]):'';
  $product = wc_get_product( $post_id );
  $product->update_meta_data( 'gizli_sku', $sku );
  $product->save();
}

add_action( 'woocommerce_process_product_meta', 'meta_kaydet' );