add-custom-promo-attribute.php

<?php

/**
 * Add the custom attribute "Special Promotion" to a product.
 */
add_filter( 'dfrpswc_product_attributes', 'mycode_add_promo_attribute', 20, 5 );
function mycode_add_promo_attribute( $attributes, $post, $product, $set, $action ) {
	if ( isset( $product['promo'] ) ) {
		$attr = 'Special Promotion';
		if ( !isset( $attributes[sanitize_title( $attr )] ) ) {
			$attributes[$attr]['name'] = $attr;
		}
	}
	return $attributes;
}

/**
 * Set value for "Special Promotion" attribute.
 */
add_filter( 'dfrpswc_filter_attribute_value', 'mycode_set_promo_attribute_value', 30, 6 );
function mycode_set_promo_attribute_value( $value, $attribute, $post, $product, $set, $action ) {
	if ( isset( $product['promo'] ) ) {
		$attr = 'Special Promotion';
		if ( $attribute == $attr ) {
			$value = $product['promo'];
		}
	}
	return $value;
}