title: Flow.js Extend imported type
body: <p>I'm trying to extend a Flow Type that I am importing from an external library like so:</p>
  
  <pre><code>import type { $Request, $Response } from 'express';
  
  
  export type Req extends $Request {
    additionalField: {
      key: string
    }
  };
  </code></pre>
  
  <p>This won't work, but I'm wondering if there is another way to accomplish this in Flow. I need a new type that inherits a previous type and has a few more properties on top of that.</p>