Here some of the solutions I found so far:
https://www.slicer.org/wiki/Documentation/Nightly/Developers/FAQ/Extensions#How_to_manually_download_an_extension_package.3F
However the site won’t load normally, see Extension app store not accessible (http://slicer.kitware.com/midas3/slicerappstore) for a workaround. Even in this case no extension is found, for any combination of OS and version…?
That site seems fundamentally broken, everything I clic just generates an endless loading in the browser.
From https://github.com/Slicer/SlicerJupyter:
jupyter-kernelspec install /tmp/SlicerJupyter-build/inner-build/share/Slicer-4.11/qt-loadable-modules/JupyterKernel/Slicer-4.9/ --replace --user
Of course the paths will have to be updated but that depends on the extension installation, which I’m not yet able to do…