MattyWjeisz Flarum uses the PHP password hashing function, so you can set a temporary password in the database by generating a bcrypt hash or other hash supported by PHP.
If you don't want to use an online generator, you can do it on the command line, for example to create a hash for password secret, you can run php -r "echo password_hash('secret', PASSWORD_DEFAULT) . PHP_EOL;". Then store that value in the password column of your user on the users table. Do not use that command with your final password though, because it will be logged in your server's bash command history.
If you configure emails, you should also be able to simply reset your password. You can set mail_driver to log via the database and it will write all emails to the Flarum log file if you don't yet have a working email server.
I'm not sure how the MyBB migrator works, but this extension exists to support migration from different hashing algorithms. It requires the old hash to be stored in a special column with a flag that indicates its format https://discuss.flarum.org/d/8631-old-passwords
