Oh gosh. To able to "hack" Twitter would be close to impossible, probably, even for the best computer hackers out there.
Your best bet would probably be some social engineering "hacks".
I know that on Twitter, it's against their ToS to squat on inactive usernames. If you want a particular username, I would email Twitter bis dev directly and ask them if you can have the username yourself. If you have a domain name registered, or better yet, a website indiciating an exact match for the Twitter username, you should be able to get it pretty easily..