krause How do we install these additional files when we want to use the plugin mod_geogebra?
Just follow step 3 here. Or maybe I did not understand what you mean.
Do you want to preinstall these files on every new MoodleBox?
As long as the plugin has not been updated, it has no sense to install this in the image. Maybe later, I've not decided yet and must think more about it: the whole library is quite heavy (ca. 100 MB!) and it's a corner case (even if I'm a mathematician).
krause Or should there be a new global geogebra plugin to install the offline support for all geogebra plugins?
A possible solution, but IMHO not needed. The plugin must just enable to set the codebase path, that's all. Then, when somebody wants it to be elsewhere, e.g. locally (but this could be anywhere, see this doc), it has to set the adequate paths.