Can I modify the source code of Bitcoin to generate addresses people have already used? Each time you start bitcoin-qt.exe program will generate new random address

But what happen if I modify source code and compile new version:


instead of new random address I can hard code in source code address which already exist on the network with BTC on it ( for example http://blockchain.info/address/1933phfhK3ZgFQNLGSDXvqCn32k2buXY8a )


After I sync "new" wallet with the chain I can retrieve all BTC on it


to do the same I can also modify RANDOM generator source code to generate any address which already exist on the network with BTC on it as above


So the question is:

It's possible to modify source code, compile to EXE and generate THAT way existing BTC address with private keys inside wallet.dat (I believe that private keys are created FROM BTC address by bitcoin-qt.exe) to steal the money
No, because you need the private key to spend the bitcoins. The chance you guess a private key is really, really, really small. 

The amount of private keys is 64^16, which is 7.9228163*10^28. Written down this number looks this way:

79,228,163,000,000,000,000,000,000,000

Just to put this number into perspective, the US debt at this moment is:

17,193,871,000,000 $

If you would be able to guess and check 100 million private keys every day, it would take you (64^16)/100,000,000=7.9228163*10^20 days, which is 2.1706346*10^18 years. That is 2,170,634,600,000,000,000 years.

The approximate age of the earth is estimated at about 4.54*10^9 years, which is 4,540,000,000 years.