Use the DateTime.Parse() method:
var datePickerDate = dateTimePicker.Value;
var comboboxTime = comboBox.SelectedText;
var dateTimeString = String.Format("{0} {1}", datePickerDate, comboboxTime);
var combinedDate = DateTime.Parse(dateTimeString);
I'm not sure if you are writing a windows or web application, but the solutions are the same. Also you might want to use one of the overloaded versions of DateTime.Parse() to have more control of the format of the stringDateTime.