Take a CSV file of date and GBP, and add the GBP=>EUR rate and converted EUR amount.

gbp2eur.js

const { createReadStream, createWriteStream } = require('fs');
const { pipeline } = require('stream');
const csv = require('csv-parser');

const exchangeRatesUrl = 'https://api.exchangerate.host';

async function getExchangeRate(date) {
  const response = await fetch(`${exchangeRatesUrl}/convert?from=GBP&to=EUR&date=${date}`);
  const { result } = await response.json();
  return result;
}
async function* addEurValue(transformStream) {
  for await (const row of transformStream) {
    const date = row.date;
    const gbp = parseFloat(row.GBP.replace(',', '.'));
    const exchangeRate = await getExchangeRate(date);
    row.rate = exchangeRate;
    row.EUR = (gbp * exchangeRate).toFixed(2);
    yield row;
  }
}
async function* csvStringify(transformStream) {
  yield 'date,GBP,rate,EUR\n';
  for await (const object of transformStream)
    yield `${object.date},${object.GBP},${object.rate},${object.EUR}\n`;
}

async function transform(inputFile, outputFile) {
  pipeline(
    createReadStream(inputFile),
    csv(),
    addEurValue,
    csvStringify,
    createWriteStream(outputFile),
    (err) => {
      if (err) {
        console.error('Error:', err);
      } else {
        console.log('Transformation complete');
      }
    }
  );
}

const [, , inputFile, outputFile] = process.argv;

transform(inputFile, outputFile);

Just npm i csv-parser, then run node gbp2eur.js in.csv out.csv.