You can create a custom RssChannelProvider that inherits from Telerik.Cms.Engine.RssChannelProvider or one of the built-in providers like Telerik.Blogs.Rss.RssChannelProvider and override CreateDataSource() method where you can use GetContent method of ContentManager class and filter the list by all categories you have.
Basically rss works as follows - you add a category, then the provider is initialized and settings are created in RssViewControl. There is a method SaveSettings() in RssSettignsControl which persist the data and
sets RssChannelProvider.KeyFilterExpr to the selected value of categories.
The actual filtering should be performed under the class that inherits from RssChanelProvider. You should pass the settings in Initialize and then override CreateDataSource and filter the data.
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