Howto use the "Just Report It" extension in Icedove like I did it in Thunderbrd before for reporting Spam to SpamCop and others?

If this is the add-on your are referring to, it is indeed released under the GPLv3 and the source code is available:

https://addons.thunderbird.net/en-US/thunderbird/addon/just-report-it

You can simply download it from that page, in Icedove go to "Add-ons and Themes" then click on the wheel, select "Install Add-on From File" and select the .xpi file you have just downloaded. Make sure you have updated Icedove to its current version (115.4).

Source code: https://github.com/justreportit