As a workaround I now use the windows' explorer side by side with ompt to drag and drop files from multiple locations into the song.
I didn't know it was even possible until a few days ! The problem is that I can't preview the sounds.
So let me advocate again for a real tree view where the hard drive would be presented as a tree of expandables folders.
As of now the tree view is not really a tree view is it ?
As hard drives get bigger and files and instruments increase in number, a directory tree browser would be the definite tool for sound design and fast workflow. I think