pewc_item_params.php

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

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

/**
 * Add the new params to the cart item data
 */
function prefix_end_add_cart_item_data( $cart_item_data, $item, $group_id, $field_id, $value ) {
	if( ! empty( $item['sku'] ) ) {
		$cart_item_data['product_extras']['groups'][$group_id][$field_id]['sku'] = $item['sku'];
	}
	return $cart_item_data;
}
add_filter( 'pewc_filter_end_add_cart_item_data', 'prefix_end_add_cart_item_data', 10, 5 );

function prefix_itemmeta_admin_item( $list_item, $field, $price ) {
	if( ! empty( $field['sku'] ) ) {
		$list_item .= '<br>' . $field['sku'];
	}
	return $list_item;
}
add_filter( 'pewc_itemmeta_admin_item', 'prefix_itemmeta_admin_item', 10, 3 );