There are a few possibilities, here. It would be very nice to know more about the radio, and what components he had available. Today, a hack can mean replacing a lot of stuff in the circuit, but somebody in Bulgaria in WWII might not have had a parts bin. Is "channel filtering" in the original text? That sounds like the radio was likely not one of the Volksempfänger type radios used in Germany during the War: they have been described as being manufactured to only receive Hitler's broadcasts, but that's not true- they were just cheap rather simple radios that could not pick up faint signals, so when the British boosted their transmission power, the radios could hear their broadcasts as well. "Channel filtering" sounds more like a super-heterodyne ( or super-het) radio, which by 1940 would be more expensive and perhaps more common, even in Bulgaria. They had more filter stages.

It'd be a fun question to post on a Ham radio forum, and you'd get much more specific hypotheses from them...maybe somebody has a Bulgarian radio circa 1940 in their collection. But, generally, a radio uses resonant circuits to tune in to the various broadcast frequencies, and then amplifying circuits to make them heard through the speaker. Adding a stage to the amplifier would have required adding some components like a tube, and a tube socket...might have been hard to do. He could have improved the antenna, to be able to pick up distant , fainter signals better. Hacking the resonant circuits ( including the filters) to change their resonant frequencies and pick up different broadcast frequencies might have been done by re-winding or replacing the inductors, the coils, to have a different inductance. For that, the hacker might have simply needed some wire. The ability and time to do some calculations - maybe a slide rule- would have been quite handy too, of course.