Adding a Grid component in the ComboBox item template will require further custom logic for changing the ComboBox value based on the Grid selection, as the whole Grid will be treated as only one data item. Basically you will have a ComboBox with 1 item that renders a custom Grid in the template. This Grid will need to be bound to its own set of data items, and when an item is selected, you can perform some custom logic for adjusting the ComboBox value.