I assume that by TextBlock you mean TextBox and that you want a single TextBox at the top of the dropdown of the RadComboBox. I'm clarifying this as placing the TextBox in the ItemTemplate of the control will add a separate TextBox for each item which does not seem desirable.
With this said, I've prepared a small sample project which I believe achieves your requirement. For the purpose, I've added a TextBox in the NonEditableComboBox template of the control and handle its TextChanged event. In it, I use the FilteringBehavior of the control to get the proper item to select based on the inputted text.
For this to work as expected, as normally the FilteringBehavior is only used when the IsEditable property is True, I'm calling the EnsureFilteringBehavior method of the control via reflection.
Please have a look and let me know if a similar approach would work for you. Alternatively, you can extract this in an attached behavior and add the TextBox via code instead of modifying the control template.
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