You can't add the coins at to the system at will.
Quote:
The number of new coins generated per block gradually decreases over time. It started out at 50 BTC, but will dwindle to zero sometime in future when all 21 million coins have been generated. Fortunately, coins can be divided down to the eighth decimal place, which may prove increasingly useful if their value grows.

http://arstechnica.com/techpolicy/n...rcurrency.ars
Quote:
People can obtain Bitcoins in two ways  buying them or generating them.
To generate them, you have to run a complex math hashing algorithm, which tries to find a new bitcoin "block". Parallel computing devices  namely GPUs have shown themselves most capable for this task. In fact with modern AMD GPUs it is possible to "break even" on your hardware costs by generating Bitcoins.

http://www.dailytech.com/Digital+Bla...ticle21877.htm