pewc_end_product_extra_field.php

<?php
/**
 * Add a custom parameter to add-on field settings
 */
function demo_add_class_parameter( $group_key, $item_key, $item, $post_id ) {
	echo '<div class="pewc-fields-wrapper pewc-my-new-fields">';
		echo '<div class="product-extra-field-third">';
			$class = isset( $item['class'] ) ? $item['class'] : '';
			echo '<label>Class</label>';
			echo '<input type="text" class="pewc-field-class" name="_product_extra_groups_' .esc_attr( $group_key ) . '_' . esc_attr( $item_key ) . '[class]" value="'. esc_html( $class ).'">';
		echo '</div>';
	echo '</div>';
}
add_action( 'pewc_end_product_extra_field', 'demo_add_class_parameter', 10, 4 );

/**
 * Register the custom param to field data
 */
function demo_register_class_parameter( $params, $field_id ) {
	$params[] = 'class';
	return $params;
}
add_filter( 'pewc_item_params', 'demo_register_class_parameter', 10, 2 );

/**
 * Filter field classes
 */
function demo_add_class_to_field_wrapper( $classes, $item ) {
	if( ! empty( $item['class'] ) ) {
		$classes[] = esc_attr( $item['class'] );
	}
	return $classes;
}
add_filter( 'pewc_filter_single_product_classes', 'demo_add_class_to_field_wrapper', 10, 2 );