From reasonable times ago (think it was Kicad V5) I remember a Austrian user who proceeded with efforts to import Altium projects. For V8 menue shows CadStar, Eagle and EasyEDA. Also KiCad official repository https://repository.kicad.org/repository.json does not know anything about this project. What happend with that?
Hahaha.
The importer is integrated into the main code and not a plugin.
and search for altium.
PS: I think the reason you can’t see it is because you have to import from the schematic and layout editors, not the project manager.
Found, its *.SchDoc direct not complete project