function merge_additional_checkout_data( $customer_data, $order ) {

    $user_id = $order->get_user_id();

    $customer_data['billing_employer'] = get_user_meta( $user_id, 'billing_employer', true );
    $customer_data['billing_occupation'] = get_user_meta( $user_id, 'billing_occupation', true );

    return $customer_data;

}

add_filter( 'wpf_woocommerce_customer_data', 'merge_additional_checkout_data', 10, 2 );