The configuration is stored in the Postgres-database that comes with the system. If you examine the configuration backup, you'll notice that it's an XML dump of that database inside a zip file. Editing this XML file is possible, but the results are questionable to say the least (since the internal config format is not documented). That said, configuring 3CX through the web interface is easy, so it would seem futile to invest time in an automatic configuration (and then there's no way to automatically configure your own sip accounts anyway).