If you need to whitelist or block a range of IP addresses using CSF, it can be done, but you need to use the CIDR format.
For example, to block 192.168.0.0 to 192.168.0.255 you would add 192.168.0.0/24 to the blocked list.
If you are unsure how to convert a range of IP’s to CIDR, I find the following site helpful. You enter the starting IP, and ending IP, and it will create the CIDR entries for you.
http://ip2cidr.com