Add custom fields under Billing Fields and save in Order Meta

function comcast_woocommerce_billing_fields($fields)

{

    $fields['gx_name'] = array(

        'label'       => __('Billing Contact for GXS', 'woocommerce'), // Add custom field label

        'placeholder' => _x('', 'placeholder', 'woocommerce'), // Add custom field placeholder

        'required'    => true, // if field is required or not

        'clear'       => false, // add clear or not

        'type'        => 'text', // add field type

        'class'       => array('gx_name')    // add class name

    );

    $fields['gx_email'] = array(

        'label'       => __('Billing Contact Email Address', 'woocommerce'), // Add custom field label

        'placeholder' => _x('', 'placeholder', 'woocommerce'), // Add custom field placeholder

        'required'    => true, // if field is required or not

        'clear'       => false, // add clear or not

        'type'        => 'text', // add field type

        'class'       => array('gx_email')    // add class name

    );

    return $fields;

}

add_filter('woocommerce_billing_fields', 'comcast_woocommerce_billing_fields');

function comcast_save_extra_checkout_fields( $order_id, $posted ){

    if( isset( $posted['gx_name'] ) ) {

        update_post_meta( $order_id, '_gx_name', sanitize_text_field( $posted['gx_name'] ) );

    }

    if( isset( $posted['gx_email'] ) ) {

        update_post_meta( $order_id, '_gx_email', sanitize_text_field( $posted['gx_email'] ) );

    }

}

add_action( 'woocommerce_checkout_update_order_meta', 'comcast_save_extra_checkout_fields', 10, 2 );