If there is nothing preventing you from running the code externally, then just put this code into another macro enabled workbook OR run it from a seperate C# winform/console application exe.
Note I had quick search of the Excel 2007 object model and didn't find the EnableGrouping method, but maybe i wasn't looking hard enough.
External VBA
Sub UpdateWorkbook()
Constant workbookpath As String = "C:\somepath\someworkbookname.xlsx"
Dim wkbk As Workbook
Set wkbk = Application.Workbooks.Open(workbookpath)
Dim wksht As Worksheet
Set wksht = wkbk.Worksheets.Item("sheetname")
wksht.EnableGrouping
wksht.Protect UserInterfaceonly = true
Set wksht = Nothing
Set wkbk = Nothing
End Sub
External C#
public void UpdateWorkbook()
{
const string workbookpath = @"C:\somepath\someworkbookname.xlsx";
Excel.Application xlApp = New Excel.Application();
// To use the next line you need the Excel Extension library otherwise use Type.Missing.
Excel.Workbook wkbk = Application.Workbooks.Open(workbookpath);
Excel.Worksheet wksht wksht = (Excel.Worksheet)wkbk.Worksheets.get_Item("sheetname");
//Check the way this method works..
wksht.EnableGrouping();
// UserInterfaceOnly is the 6th parameter, so 5 before and 11 after.
wksht.Protect(missing,missing,missing,missing,missing,True,missing,missing,missing, missing,missing,missing,missing,missing,missing,missing,missing);
Set wksht = null;
Set wkbk = null;
Set xlApp = null;
}