Try adding one and then check your .htaccess file and see what changes were made you could then use Notepad++ or something similar to copy and paste the other IP's in and add the extra code you need for it to work. Then simply save and re-upload to your server.

This could slow down your server since it will check every user who comes to your site against the blacklisted IP's to see if any match so it could potentially cause some lag depending on how busy your site is.

JGM.