Of course FreeBSD supports multiple devices. 🙄
pfSense is a firewall/router and was never intended to support multiple storage devices other than as a drive mirror. That's not to say it can't be done. If you search the forum there are multiple threads with users describing their own solutions for making it work. However all of them operate outside the default pfSense config such that if you need to reinstall and restore for example you need to be sure you can restore any custom scripting you added.
It's far simpler to just install and boot from the larger drive directly.