It's not clear what you mean and how you added the input row.
Do you want to sort grid rows interactively by clicking column headers but keep the input row at the top after sorting? If you make the first row frozen, you will get this behavior. It's even a preferable way to make this row always visible and accessible for the user regardless of the scrolling position. We demonstrate this in the My Customers sample in the main iGrid.NET demo.
An alternative way to exclude a row from sorting but leave it scrollable is to set the row's Sortable property to False.