Not necessarily a catch though. Many (most?) registrars and web hosts provide name servers free for their users (GoDaddy lets you do this for example). If you can add/edit DNS records can point a subdomain anywhere you like.
You can do that, but nobody believes your nameserver is authoritative for questions about google.com, so it will only affect someone whom you can persuade to use yours as their default nameserver. Everyone else who asks the "com" top-level domain nameservers who is authoritative for google.com is going to be sent to GOOG's nameservers, not yours, so names you define will be invisible to them.
