Forums module is not Generic Content based module and its categories are set per forum. You can categorized and not categorized forums. For posts you need to implement a custom data layer and data provider that will allow you to set category per post/thread.
1. Implement a custom data layer by using OpenAccess ORM, LINQ -SQL or another technology you prefer.
2. Create a custom class that inherits from Telerik.Forums.Data.DefaultProvider and implement a method that interacts with your custom data layer.
3. Modify backend and front end backend templates Sitefinity/Admin/ControlTemplates/Forums and Sitefinity/ContrrolTemplates/Forums and implement logic that will call the methods from your data class.
Another option is overriding the public controls from Telerik.Forums.WebControls namespace /
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