title: Modify PayPal Express Checkout custom parameter after payment
body: <p>I'm using Paypal Express Checkout, for recurring payments without any trial.</p>
  
  <p>For the first payment, I'm using <code>DoExpressCheckoutPayment API Operation (NVP)</code>, and store reference value in custom parameter.</p>
  
  <p>Before making payment, I don't have a customer ID, as it is generated after payment is made, so I'm using the reference value in custom parameter for validating payments and register customer later</p>
  
  <p>Is there a way to modify the custom parameter later, which was set in <code>DoExpressCheckoutPayment</code>?
  I want to add the customer id to it, to ease the future IPN, as reference ID might not be available later.</p>