I'm probably not the best person to try to explain this, since I have never tried to use KSLC for this purpose, but I believe the trick is using the "duplicate screens" feature to move the source screens to their intended position in the target level, so all the metadata (shift, warps, etcetera) in the World.ini file is automatically adjusted and you can import it easily into the new file. However, if you're not using any shifts/warps in the original level (or if you just don't need them in your new level), you could just open two different instances of the KS+ Level Editor and start copy/pasting screens from one editor window to the other. You may also need to adjust the numeric indexes (tileset A/B, background...) afterwards, if you're using custom assets.