https://www.freshports.org/net/amnezia-kmod/ (https://www.freshports.org/net/amnezia-kmod/)
https://www.freshports.org/net/amnezia-tools/ (https://www.freshports.org/net/amnezia-tools/)
Even if FreeBSD supports it in kernel mode, does it still require separate work on the OPNsense side?
If it works in FreeBSD you can build the binary and use it.
If you mean if there will be a GUI for it, that requires work and somebody has to spend time to do it.