Can you share a little bit around what you are trying to accompolish? Inheriting from a Kentico class is just like any other.NET class. There are a ton of examples in the documentation around this.
If you are trying to extend the functionality or change it, the Kentico recommended best practice is to inherit the class and use the OVERRIDE call to perform your additional functionality. Make use of look into the documentation to learn about calling he base function first (the function in the original class) and then adding your additional code.