I agree with warpstorm. However, if you attack a civ which has more progresses than you, it should be possible. Lets take an example.
Your civ have barely reached the Industrial Age (no tech discovered yet). The civ you attack discovered up to Communism, Espionage, The Corporation, Electricity and Sanitation (9 techs you don't have). You capture the first city, then a second and a third. The fourth did have a couple of Ironclads. Yes, you do distroy them, but I don't think that in RL, you wont study them. So, you will be likely to discover Steam Power, especially if you are researching it. Then, 10 cities later (you discovered Medecin 6 turns ago), you capture your 6th city with an Hospital. Despite you are researching Electrcity, you discover Sanitation, thanks to this conquest.
What I'm trying to say, is that the more you are experiencing an unknown tech (in battles or city conquests), the more likely you are to discover a tech, especially when you are researching for it. And by the way, I believe that every city of your ennemy are aware of the techs they have discovered (they may have plans); so in interrogate the citizens, you might get some information, which will make you even more likely to discover by conquest.
It's just like if China had discovered something (lets say a gadget), but disn't communicate about it abroad. If you travel to China, you will be likely to discover this gadget and take one home.
