To achieve your requirement, I would suggest creating a custom style for the RadSlider control and removing the Disabled states or the IsEnabled triggers from the control templates of its elements. Please note that the exact modifications which will be required depend on the exact theme you're using and whether you've used the implicit styles approach or the StyleManager theming mechanism.
For you convenience, I've prepared a small sample project where I've removed the Disabled states for the control in the Office_Black theme with the standard Xaml binaries.
If you require any assistance in adapting this approach in your setup, please specify the exact theme(s) you're using as well as the theming approach you've chosen (implicit styles with NoXaml binaries or StyleManager with Xaml binaries) and I will gladly help you out further.
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