Adding Menu Items using the BlackBerry WebWorks SDK is well documented.
My Question how to add a Submenu-Item?
Creating a submenu is documented only for developing UIApplication
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That cannot be done in WebWorks with the available API's, however, you could always create a custom extension if it's something that you really need for your application.
Check-out this documentation if you're interested: https://developer.blackberry.com/html5/documentation/using_javascript_extensions_1866976_11.html