If you export your settings, edit the file such that “followed_communities” has all communities you want to be subscribed to, then (wait so it doesn’t give you an error, then) import the edited file, it should have you subscribed to all the communities you added.

To get all communities I’d use the api, i.e. https://hexbear.net/api/v3/community/list?type_=Local&limit=50&page=1, then copy the raw text into https://regexr.com/, make the regex "actor_id":"(.*?)" and use the List tool with "$1", (see screenshot), copy the output into a text file, open the next page on the api and go on like that until the api returns blank.