To import/export related product values based on product SKUs using WP all import/export (https://wordpress.org/plugins/wt-woocommerce-related-products/)

functions.php

<?php //do not copy this line of code

add_filter('wp_all_export_csv_rows', 'wt_process_all_export_export', 999999, 3);
function wt_process_all_export_export($rows, $export_options, $export_id) {
    $product_skus = array();
    foreach ($rows as $r_key => $row) {
        foreach ($row as $d_key => $d_value) {
            if ($d_key == '_crp_related_ids' && !empty($d_value) && (@unserialize($d_value) == false)) {
                $s_value = explode(',', $d_value);
                if (!empty($s_value)) {
                    foreach ($s_value as $key => $id) {
                        $product_sku = get_post_meta($id, '_sku', true);
                        $product_skus[] = $product_sku;
                    }
                }
            }
        }
        if (!empty(array_filter($product_skus))) {
            $rows[$r_key][$d_key] = implode(",", $product_skus);
        }
        unset($product_skus);
    }
    return $rows;
}


/*To import related product values based on product SKUs based using WP all import*/

add_action('pmxi_update_post_meta', 'wt_process_all_import_imports', 99999, 3);
function wt_process_all_import_imports($pid, $meta_key, $meta_value) {

    if ($meta_key == '_crp_related_ids') {
        $custom_meta = explode( ",", $meta_value );
        foreach ($custom_meta as $sku) {
            $product_ids[] = wc_get_product_id_by_sku(trim($sku));
        }
    }
    update_post_meta($pid, $meta_key, $product_ids);
}