This should work, but it's going to require Bash, which you can get from a stock Cygwin installation.
ls | while read csvfile
do
IFS=, read olddate open high low close volume < <(tac $csvfile)
read newdate < <(date -d "$olddate + 1 day" +%Y%m%d)
echo $newdate,$close,$close,$close,$close,$volume >> $csvfile
done
Getting Yesterdays or Tomorrows Day With Bash Shell Date Command