You can certainly make it so that people can use the free subdomain to reach your website. Whether that would actually circumvent filtering systems is a different story and will largely depend on how the filtering system works.
To allow people to access your website using the subdomain URL, you'd just register the subdomain you want, and they will ask you for the IP address you want it to point to. Of course, here you would enter the IP address of your web server.
If your website is the only site on the webserver, then this would probably be all you'd have to do. But if there are multiple sites on the server, you will need to add a VirtualServer entry for the subdomain in your webserver configuration so that the webserver knows which website users are trying to reach when they access the webserver's IP. If you are using shared hosting you can probably ask your hosting provider to do this for you.
But like I said earlier, whether this would actually circumvent filtering depends on how that filtering system works. If it filters based on domain name, then there's a good chance it would work, but if it filters based on IP address, it would do no good because your website will be at the same IP address no matter how many subdomains you register for it.