How to add `mozSystem` to XHR object?

Mozilla has two custom extensions which can be added to the `XMLHttpRequest` object: `mozSystem` and `mozAnon`.

`mozSystem` is particularly useful because it disables the Same Origin Policy on Firefox OS applications. This allows FFxOS mobile devices to avoid preflight `OPTIONS` requests. The full documentation on this property states:

> Setting this flag to true allows making cross-site connections without requiring the server to opt-in using CORS.

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest

Is there a way to add `mozSystem: true` to the underlying `XMLHttpRequest` object built up by `Superagent`?