使用Open XML SDK直接将XLSX文件转换为XLSM格式，无需使用Google电子表格进行转换。

示例代码如下：

using (SpreadsheetDocument document = SpreadsheetDocument.Open("input.xlsx", true))
{
    WorkbookPart workbookPart = document.WorkbookPart;
    if (workbookPart != null)
    {
        foreach (WorksheetPart worksheetpart in workbookPart.WorksheetParts)
        {
            if (worksheetpart != null)
            {
                string sheetName = worksheetpart.Uri.ToString().Replace("/xl/worksheets/sheet", string.Empty).Replace(".xml", string.Empty);
                Worksheet worksheet = worksheetpart.Worksheet;

                // Do any necessary modifications to the worksheet here

                // Save the modifications to the worksheet
                worksheet.Save();

            }
        }
    }

    // Convert the XLSX file to XLSM format
    workbookPart.Workbook.SaveAs("output.xlsm");
}