unbounded ah, my apologies for misunderstanding. You have an existing SSL certificate and you want to instruct FreeFlarum to use it.
Unfortunately, this is not possible in settings. However, if you send me the certificate .pem and .key (info@freeflarum.com from your admin email), I am willing to instruct FreeFlarum to use it manually. But note that we do not officially support this kind of setup and are not responsible if anything goes wrong.