Olá, boa tarde!
Você pode recuperar o json do pedido pela seguinte api https://{accountName}.{environment}.com.br/api/oms/pvt/orders/orderId
Nela existe o campo paymentData e depois connectorResponses... e lá consta o nsu.
"payments": [
{
"id": "xxxxxxxxxxxxxxxxxxxxxxxx",
"paymentSystem": "2",
"paymentSystemName": "Visa",
"value": 199,
"installments": 1,
"referenceValue": xxxxx,
"cardHolder": null,
"cardNumber": null,
"firstDigits": "xxxxxx",
"lastDigits": "xxxx",
"cvv2": null,
"expireMonth": null,
"expireYear": null,
"url": null,
"giftCardId": null,
"giftCardName": null,
"giftCardCaption": null,
"redemptionCode": null,
"group": "creditCard",
"tid": "xxxxxxxxxxxx",
"dueDate": null,
"connectorResponses": {
"Tid": "xxxxxxxxxxxxxxxxxxxxxxx",
"ReturnCode": "xx",
"Message": "Sucesso",
"authId": "xxxxxxx",
"nsu": "00000000000"
},