Adding extension to Selenium2(WebDriver) chrome driver

I'm using the code below to start chrome using webdriver (selenium 2)

            Map<String, String> mobileEmulation = new HashMap<String, String>();
            mobileEmulation.put("deviceName", "BlackBerry PlayBook");

            Map<String, Object> chromeOptions = new HashMap<String, Object>();
            chromeOptions.put("mobileEmulation", mobileEmulation);
            DesiredCapabilities capabilities = DesiredCapabilities.chrome();
            capabilities.setCapability(ChromeOptions.CAPABILITY, chromeOptions);
            driver = new ChromeDriver(capabilities);

How can I also load a Chrome extension while also keeping the options above?