On Linux, Mac, or if you are running under Cygwin on Windows, you can issue the following with bmwconv to process all files under the current directory:
find . -type f -iregex ".*[.]br[3-5]$" -print0 | xargs -0 -n 1 bmwconv
Make sure that you run that from the directory in which bmwconv resides or change the path to bmwconv.
Keep in mind bmwconv converts to/from. If you run it once, it converts to, twice, it converts from. Running it twice gives you what you started with.