I don't know how it's done, and the people who do know how it's done are unlikely to tell you (decryption methods for digital satellite are not generally discussed in open forums). I believe it used to be possible to capture Sky Digital using certain DVB-S cards to which you could attach a conditional access module (CAM) emulator.
What I'm basically saying is that if you do want to go ahead with this, be prepared to spend a lot of time carrying out difficult research, and a lot of money on specialist hardware. Good luck!