Получил именно то, что мне нужно:
public static SpreadsheetGear.IWorkbook GetObjectFromS3AndReturnWorkbook(string key, string path, string bucketName)
{
GetObjectResponse response = client.GetObject(bucketName, key);
MemoryStream memoryStream = new MemoryStream();
using (Stream responseStream = response.ResponseStream)
{
responseStream.CopyTo(memoryStream);
}
IWorkbookSet workbookSet = Factory.GetWorkbookSet();
SpreadsheetGear.IWorkbook workbook = workbookSet.Workbooks.OpenFromStream(memoryStream);
return workbook;
}