Thanks for contacting us and for the nice words about RadControls for Windows Phone!
Currently the GroupPicker of RadJumpList cannot be designed within Expression Blend. Since, however, the component is simply a popup with an Items Control that positions items according to its layout panel, you can use a standard ItemsControl in expression blend and design its appearance and reuse it for the group picker.
Let me know in case you need further assistance.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>