To add a new sheet by the name of "NewestSheet" to the workbook "D:\XLSFiles\C.XLS" just before the sheet named "MyNewSheet" the following syntax would be used:
ConvertXLS.EXE /S"D:\XLSFiles\C.XLS" /T"D:\XLSFiles\C.XLS" /P104 /1"NewestSheet" /2"MyNewSheet" /4TRUE
Note: You don't have to specify Boolean (TRUE/FALSE) parameters that are FALSE. Nor do you have to specify parameters that are empty.
The /S and /T switches specify original and target workbook respectively, which in this case is the same workbook. You can optionally keep the original file unchanged by specifying different files for the original and target.
In this example, notice that even if "NewestSheet" already exists, it will be completely overwritten by a new empty sheet.