I might have misunderstood the use of the extension.
I imagined that I could export the json file from a site and import it on a new site where metabox is installed....
After reding your comment I think that is nont possible, right?
Yes, it's not possible to import ACF json to Meta Box. But you can install the plugin Meta Box and MB AIO on that site, and migrate the data to the Meta Box. Then export the field group from that site and import it to the new site.