I was almost there. I wasnt building the single js file properly.

src/index.js

var convert = require("xml-js");

module.exports.data_process = function (inJson) {
  var xml = "<XML CONTENT>";
  var result1 = convert.xml2json(xml, { compact: true, spaces: 4 });
  var parsed = JSON.parse(result1);
  var some_field = parsed.some.field
  var obj = JSON.parse(inJson);
  obj.some_field = some_field;
  return JSON.stringify(obj);
};

webpack.config.js

const path = require("path");

module.exports = {
  entry: "./src/index.ts",
  mode: "production",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    library: "xmllib",
  },
};

build webpack

npx webpack --config webpack.config.js

Then add to the end of main.js

function process(d) {
    c = xmllib.data_process(d);
    return c;
}

Now you can use "process" as your dataflow udf function name.

Solution based on this article: https://blog.salrashid.dev/articles/2020/bq-udf-protobuf/