Custom sounds are limited to Music, Ambiance, Shifts, and Triggers. I know it would be cool to customize any sound the way you can customize any graphic, but it's beyond the scope of this project. Also, I have concerns over the performance of non-embedded sounds.
KS+ is meant for novice level designers. I consider sound effect design to be an intermediate level skill. I'd also put Lua programming in the intermediate zone, so if you have any programming knowledge
KS Ex may be a better fit for you.