How can I monkey-patch a module in typescript

Question

I'm just learning typescript. In javascript, I use bluebird to promisifyAll fs, using a version of fs.existsAsync from https://github.com/petkaantonov/bluebird/issues/418:

import * as Promise from 'bluebird'
import * as fs from 'fs'

Promise.promisifyAll(fs)

fs.existsAsync = Promise.promisify(
  // istanbul ignore next
  function exists2 (
      path : string,
      exists2callback : (err:null, exists: boolean) => void) {
    fs.exists(path, function callbackWrapper (exists) {
      exists2callback(null, exists)
    })
  })

However, typescript complains:

error TS2339: Property 'existsAsync' does not exist on type 'typeof "fs"'.

Is there a best practice to get around this?