wc-sku-generator-change-overall-sku-format.php

<?php // only copy if needed

/**
 * Adjust the format of generated SKUs
 *
 * @param string $sku the original complete SKU
 * @param string $product_sku the parent product SKU
 * @param string $variation_sku the SKU to represent a variation, if set
 * @return string the updated complete SKU
 */
function sv_wc_alter_generated_sku( $sku, $product_sku, $variation_sku ) {
	return $product_sku . $variation_sku;
}
add_filter( 'wc_sku_generator_variation_sku_format', 'sv_wc_alter_generated_sku', 10, 3 );