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Although confusing, the FAQ in the ui-router wiki seems to say that this isn't possible: ui-router/wiki/faq
One approach is to allow each feature to define it's own root state (as in this example: AngularJS State Management with ui-router)
Another would be to define the entire state hierarchy in your "myApp" module and just leverage controllers etc. from the dependent modules. This works especially well when you are maintaining a mobile and desktop site; define each site's state hierarchy in a mobileApp and desktopApp module, and have the functionality (e.g. controllers, services, etc.) shared in separate modules.