Hi,
Thanks for your interest in Syncfusion Essential Studio.
You cannot add a DateTime control to the ComboBoxBarItem. But you can add any control to a PopupControlContainer and attach that to the DropDownBarItem such that it looks like a DropDownDateTimePicker.
Please refer to the below sample on how you can make the same. Please try this and let mek now if this helps.
http://www.syncfusion.com/uploads/redirect.aspx?file=85954_WinApp_aa3a05b0.zip&team=supportRegards,
Nanda