Your main problem is giving the Server the illusion that there's a human on the other side. If the game you want to support doesn't allow client-side modifications then you probably need to do lots of hacking.
Usually client-bots work by interpreting and modifying the network pakets send forth and back between client & server. I guess this would be what you would have to do too. Messing with this, I'm afraid leads directly into issues considered with cheating/anti-cheating. If you're doing this out of interest for client-side things then I'd choose a game where this is already possible through 3rd-party addons. For example there's a "Gamebot" interface for UT 2004 and I believe there has been something like that for Neverwinter Nights too. I doubt that many people here can help you because these forums are very Valve Engine-centric