[pkscout]

Many of my artists have no extrafanart (CDart MAnager plugin does not find any pictures) while this script (Artist Slideshow) displays many images for these artists.
I thought that progressively, I could fill my empty extrafanart folders.

You can do this manually. The cached files are stored in userdata/addon_data/script.artistslideshow/ArtistSlideshow. The directory names are hashed, but you can look at the images to see which artist is in what folder. Copy the tbn files to a new extrafanart directory and rename them as .jpg files.

It is unlikely that doing this automatically will be added. The names of the folders and files are hashed, I suspect, to avoid problems with odd characters in artist names and platform specific characters not allowed in names. I can use an existing directory because I know the name of the directory must be OK because it exists.