MailPoetには標準でWooCommerceと連携する機能がありますが、
Welcartはなかったので、作ってみました。
最初は$wpdbで直接テーブルにinsertしてましたが、
ウェルカムメールが送信されないなどの問題があったので、プラグインのAPI経由で登録します。
https://github.com/mailpoet/mailpoet/tree/master/doc

$mailPoetIntegrator = mailPoetIntegrator::getInstance();

class mailPoetIntegrator
{
  private static $instance = [];

  private $mailpoetApi;
  const SEGMENT_PURCHASE = 【wp_mailpoet_segmentsのid】; //購入者用のメルマガリスト
  const SEGMENT_REGISTER = 【wp_mailpoet_segmentsのid】; //会員登録者のメルマガリスト

  private function __construct()
  {
    if ( ! class_exists(\MailPoet\API\API::class)) return;

    $this->mailpoetApi = \MailPoet\API\API::MP('v1');

    add_action('usces_action_after_send_ordermail_to_manager', array(&$this, 'completionPurchase'), 10, 4);
    add_action('usces_action_member_registered', array(&$this, 'registeredMember'), 10, 2);
  }

  public static function getInstance()
  {
    $class = get_called_class();
    if (!isset(self::$instance[$class]))
      self::$instance[$class] = new $class;

    return self::$instance[$class];
  }
  public final function __clone()
  {
    throw new \Exception('Clone is not allowed against' . get_class($this));
  }

  private function insertMember( $email, $first_name, $last_name, $segment_id )
  {
    try {
      $this->mailpoetApi->addSubscriber(
        array(
          'email' => $email,
          'first_name' => $first_name,
          'last_name' => $last_name
        ),
        array(
          $segment_id,
        ),
        array(
          'send_confirmation_email' => true,
          'schedule_welcome_email' => true,
          'skip_subscriber_notification' => true
        )
      );
    } catch (\Exception $e) {
      // ログ出しとかエラーメッセージを出したければここで
      return;
    }
  }

  public function completionPurchase( $res, $bcc_para, $entry, $data )
  {
  
    if( ! $email = @$entry['customer']['mailaddress1'] ){
      if( ! $email = @$entry['delivery']['mailaddress1'] ){
        $email = @$data['order_email'];
      }
    }
    if( ! $email ) return;
  
    if( ! $first_name = @$entry['customer']['name2'] ){
      if( ! $first_name = @$entry['delivery']['name2'] ){
        if( ! $first_name = @$data['order_name2'] ){
          $first_name = '';
        }
      }
    }
  
    if( ! $last_name = @$entry['customer']['name1'] ){
      if( ! $last_name = @$entry['delivery']['name1'] ){
        if( ! $last_name = @$data['order_name1'] ){
          $last_name = '';
        }
      }
    }

    $segment_id = self::SEGMENT_PURCHASE;

    $this->insertMember( $email, $first_name, $last_name, $segment_id );
  
  }

  public function registeredMember( $post_member, $user )
  {
  
    if( ! $email = @$post_member['mailaddress1'] ) return;
  
    if( ! $first_name = @$post_member['name2'] ){
      $first_name = '';
    }
  
    if( ! $last_name = @$post_member['name1'] ){
      $last_name = '';
    }

    $segment_id = self::SEGMENT_REGISTER;

    $this->insertMember( $email, $first_name, $last_name, $segment_id );

  }

}