You can just drag and drop media to the correct Launchbox folders, no more difficult than that, you dont have to do it in the UI, just do it through explorer like you would when moving any other files. You can also if you want point Launchbox to your existing folders and it will use those, that method would be more manual work though. When you import a system Launchbox will create the media folders, regardless of if you actually specified to download any media or not. So once the roms are in you can use either method i just mentioned to use your existing media, as long as the files are using the actual game name, or the name of the actual file, Launchbox should pick up the media.