You can use Webmin for FreeBSD, but serious FreeBSD users would *never* use a graphical tool to administrate their server. The command line can be much faster, and there is less chance of things going wrong.
If you are taking the time to learn FreeBSD (a Good Thing), then I suggest you take the time to learn how to edit all the configuration files manually. Use a good text editor (ee, pico, vi), and once you know the system, you will be able to change settings faster than any other way. It's not that difficult.