I've just checked this - TC is able to access XLSM files via DDT by using the ACE driver (see the "DDT.ExcelDriver" help topic).
As for converting your files, you can open Excel via COM (Sys.OleObject("Excel.Application")), open the target file and save it in another format by using Excel's SaveAs method. See Excel's built-in VBA reference for information on its object model.
Thanks for all your answers. I'm using the DDT.EXCELDriver. And the files I'm comparing are macroenabled excel files. Yesterday, I tried to compare the files and it didn't seem like it was working. I'll try again. And thanks again for your prompt responses.