How do I extend a class definition that's missing types from DefinitelyTyped?

Question

I want to use the Driver instance method setNetworkConditions from selenium-webdriver, which is available in the source.

DefinitelyTyped is currently missing the types for this function. In fact, all it has is this:

export class Driver extends webdriver.WebDriver {
  /**
   * Creates a new session with the ChromeDriver.
   *
   * @param {(Capabilities|Options)=} opt_config The configuration options.
   * @param {(remote.DriverService|http.Executor)=} opt_serviceExecutor Either
   *     a  DriverService to use for the remote end, or a preconfigured executor
   *     for an externally managed endpoint. If neither is provided, the
   *     {@linkplain ##getDefaultService default service} will be used by
   *     default.
   * @return {!Driver} A new driver instance.
   */
  static createSession(
      opt_config?: Options|webdriver.CreateSessionCapabilities,
      opt_service?: remote.DriverService|http.Executor): Driver;
}

I want to extend this definition within my ambient.d.ts file, which I have successfully used for projects that do not have any TS definitions at all.

Now, how go about doing this? I have tried adding this:

declare module 'selenium-webdriver/chrome' {
  class Driver {
    setNetworkConditions(spec: {});
  }
}

But as soon as I add this, all other function type definitions from the Driver class that it inherits from webdriver.WebDriver appear as missing.

I have seen this question but it does not seem to match the use case of extending existing definitions.

How could I merge the definitions, or extend them, without having to copy-paste the DefinitelyTyped definitions for webdriver.WebDriver?