Is the folder a physical folder outside of WordPress? If so, then the easiest/simplest way to do what you want to do is to create/add another .htaccess file in that folder. Use Notepad or Notepad++ (Do NOT use Word or WordPad) to create the .htaccess file for this folder. Save this .htaccess file with this name - special.htaccess so that you can save the file on your computer. After you have added the directive you want below to this file and save it you will be uploading the special.htaccess file to your folder and renaming it to just .htaccess.
If you would actually like to index (show the files in that folder in a list) the files in that directory then you would use the Options directive as shown below.
# Show files in an indexed list
If you would like to turn off rewriting in this folder then use the RewriteEngine directive and set it to Off. By adding this one line of code in your special.htaccess file you are turning off security in this folder. The BPS root .htaccess file will no longer apply its rules in this folder.
# Turn Off Rewriting