ASAIK there is two way to create/import subfolders with QML :
- Import with relatives path
import "myQMLDir/mySubDir"
- Import modules
import myQMLDir.mySubDir 1.0
while you create qmldir files and add them to the import-path of the QtQuick engine.
The 1st one seems much simpler to do. The second one allows you to version your files or import them from external directories, but when you use them inside a project is it useful ?