Can you plug a ballast into a 120V outlet that is normally meant for 240V outlets if it's a 400W metal halide light bought in Korea and will it run in the US?

Well, honey, trying to plug a ballast designed for a 240V outlet into a 120V outlet is like trying to fit a square peg into a round hole - it just ain't gonna work. And let's not forget that different countries have different electrical standards, so that 400W metal halide light from Korea might just throw a tantrum when you try to run it in the US. So, in short, don't even bother - you're better off finding a compatible ballast or light for your US setup.