When using the plugin ttrss-data-migration, as of commit 6953ca02fd, the resulting JSON file doesn’t include the author field of each feed’s item nor is able to read them (I attempted to add the field manually).
i’ll make a note to take a look at this when i have some time.
i haven’t tested the import but it should work, i think: https://git.tt-rss.org/fox/ttrss-data-migration/commit/a3e78474d0084812ca50cf62cb792c186e59f1ca
It would be nice to also export the unread status of articles.