Can I use current Bitcoin protocol and Bitcoin client to create my own controlled currency?
Bitcoin is a decentralised cryptocurrency, meaning there is no central authority to control it. What you need to look into are centralized currencies. If you want to go for the easiest route, all you need is knowledge of programming websites and databases and you can easily make some database storing arbitrary numbers representing an amount of your currency belonging to someone. This, however, would be like Facebook credits, Microsoft Points or the like, not really a cryptocurrency. You could look into centralised cryptocurrencies, like loom.cc . Also worth noting are Open Transactions.

One could, however, create an alternative decentralised cryptocurrency, like SolidCoin, LiteCoin and dozen of others, but that doesn't create as much solutions as it causes problems. If you'd modify the source code you could make it so that only one address is allowed to create coins, but the whole system would be vulnerable to numerous attacks as your computing power might not be enough to secure the system.

All in all, it is not hard to create a centralised currency, but Bitcoin model might not be the best to use for this problem.