Assuming you have Excel, open office or similar spreadsheet program it seams weird to me to use a program that is not designed to handle tables of data to do what the native program is designed to do.
If the scale of the issue is about 20 files I reckon you would be quicker to just open then all with the spreadsheet and 'save as' rather than spending the same amount of time researching, asking about and writing a LISP to do that, and then there is testing afterwards. When I am working my goal of automation is to make more time in the day to get a cup of tea, not spend more time and get fewer cups.
However if you are likely to do this regularly then it might be worth writing something to do this... but is could be a slow running LISP. Unless someone has something really clever, my instinct is that AutoCAD won't be able to save an XLS file directly as CSV, you'll have to read the data to memory, create a CSV file and write the data to that. Which like you say takes time with GetExcel Lisp.
Can I ask why you want to do all this with LISP rather than with a spreadsheet?