If the RadCube is part of a UserControl (Page), you can assign them names (with x:Name="SomeName") and VS will generate fields for you that toy will be able to access from the code-behind.
If it is part of a template and you have access to the grid, you can use
TextBox myTextBox = grid.FindName("TextBoxName") as TextBox;
All the best,
the Telerik team