In general, you could take a look at Editing Control Templates
topic in our documentation which explains how you can extract and edit the template of any of our WPF controls.
However, please note that replacing the TextBox directly with MaskedDateTimeInput control is not a tested scenario and we cannot guarantee that it will work without problems - there can be unexpected results due to the specifics of the parsing logic of DateTimePicker control.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.