The functionality you described cannot be configured out-of-the-box and there is not a workaround I can suggest, because implementing such a non-linear RadSlider will require a lot of changes in the control.
There is a feedback item about creating non-linear sliders that you can use to track the status of this feature request, vote for it and comment it: http://feedback.telerik.com/Project/108/Feedback/Details/40473
. If the feature gets a lot of votes it will be considered for implementation.
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