You would need to send a spy to steal it from someone that already researched the tech. Meaning, if nobody has beaten you to for example, stealth bombers, you are unable to boost the tech at all. However, Rocketry and a few others can be boosted through this method or via Great Scientist, whichever happens first. Though by this point in the game, I'd just hard research it if you're in single player. It's not worth sending a spy to a city for the sole purpose of stealing that one tech boost over the course of however many turns, all while delaying a technology. I would recommend just researching it the hard way, rather than timing out this whole thing.
