If you are either a citizen or an overseas Korean holding F-4 visa, you just need 1000 hours total however if you only have something like 15 hrs of multi-engine piston hours, KAL doesn't look at it so favourably.
As you know, their cadets fly multi-engine piston cargo via Ameriflight therefore it's better if you have at least some experience of different types of aircrafts.
On the other hand, this is very important to you. You must find out whether you will get Level 1/2/3 or Level 4 from Korean Military Manpower Administration. Because if you get Level 4, you do your service in civilian world hence you can do some CFI flying whilst doing compulsory national service. But if you get Level 1/2/3, then you are pretty much screwed because you won't be able to fly for the next 21 months.
You must also have 4 years university degree for a job in Korea, including KAL.
Good luck.