Overriding ImportOverride in XlsxFormatProvider is not necessary. There is a static class called WorkbookFormatProvidersManager which has a method Import. Using this method you can retrieve a Workbook object and set it to your RadSpreadsheet. Here is some code you can find useful:
(FileStream fileStream = File.OpenRead(
.spreadsheet.Workbook = WorkbookFormatProvidersManager.Import(
NOTE: In order to use our XlsxFormatProvider you need to register it first using the following code:
Hope that helps. If you have any other difficulties get back to us.
All the best,