This can't be done out of the box. Here are some ways that you can achieve this:
1. You can modify the template of the control and set to its DropDownButton (x:Name="PART_DropDownButton") IsOpen = true. The setback will be that if you have more than 1 DatePicker on the page clicking on one of the dropdowns will close the other.
2. You can use DatePicker and just set IsDropDownOpen="True" on Focus.
3. You can use two controls- 1 Calendar and one RadMaskedDateTimeInput
and bind them to the same value in your ViewModel.
Hope this helps.
the Telerik team