We have Code Library article
showing you how to replace RadDatePicker's default input box with a regular ASP.NET TextBox control. Starting from that example, I have created a small page demonstrating how to replace the input control of a RadDatePicker with a RadMaskedTextBox control for which you can easily set an input mask.
The idea is to hide your default input box for the date picker, and bind your custom input control to the picker using client API. You need to handle three points of execution:
1. When the popup is displayed, we need it below the custom input control
2. When a date is selected, we need to display its value in the custom input
3. When a date is entered manually, we need to parse it and set the date picker's selected date
You can take a look at the sample page and see how this solution works for you.
the Telerik team