This can't be done by just editing the XML, you have to edit the code. As it is now the example runs the top level (or the category level) of the menu from appHomeScreen.brs then when a category is selected it loads its feed XML and displays it with appPosterScreen.brs, which displays things in a category leaf style (so the categories there run along the top of the screen) and then when something is selected there it displays it with appDetailScreen.brs. So you'd have to build in the functionality to display deeper regular categories as well as build in functionality to load in the changes to your XML files so you can define deeper categories in them.
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