tlauen FreeFlarum themselves does not develop any solutions that help you migrate to Flarum. Likewise, we unfortunately also do not support the idea of already established and running forums to migrate into FreeFlarum, as this service is primarily meant to help new and starting communities grow, and later migrate out (so forums hosted by FreeFlarum are allowed to download a backup copy of their data at anytime).
However, you may check the existing tools made by the Flarum community that may aid you in the process of converting your database and files to Flarum's format. Please, see https://discuss.flarum.org/d/31776 and https://discuss.flarum.org/?q=phpbb for more information.
Once you convert your phpBB backup to Flarum and are looking for a hosting service tailored for professional and grown communities, I recommend you to check out the Blomstra project, which is run by members of the Flarum team (side note: FreeFlarum is a community provided service, and is not affiliated with the Flarum Foundation).