Can I perform an async function within transformItems() on the hits instantSearch widget?

No. The transformItems function does not support asynchronous functions. Instead, you can perform your first request outside of the instantSearch widget, like this:

const client = algoliasearch('','');
const search = client.search;
client.search = async (queries) => {
  const response = await search(queries);
  const extraResponse = await myExtraRequest(response);
  return {
    ...response,
    ...extraResponse
  }
}