By design RadFileExplorer can work only with paths to files and folders placed inside the applciation's root directory. In order to implement the desired scenario you will need to implement a custom FileBrowserContentProvider
which allows for different file sources to be used in the RadFileExplorer control.
If you find it useful, you can use the SQL provider implemented by us as a base for your own implementation: Connect RadFileExplorer or RadEditor to a SQL database
There is also a live demo with the above content provider implemented, where you can test its behavior:
Custom File Content Provider
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