Thank you very much for the hint, Nigel.

In the meantime I bumped into another problem. I found out that
installation of an add-in is bound to the user logged-in: it is
registered in HKEY_CURRENT_USER not in HKEY_LOCAL_MACHINE.

As I intended to run the script as a scheduled task when nobody is
logged in on the computer I have great doubts whether it could work...

If you happen to have an idea how to do it anyway, please, let me know.

Regards,
Nick