FreeBSD & FIBs (setfib) - How to modify?

[Brent Bloxam]

The documentation on FIBs is pretty slim unless I've been looking in the
wrong places, all I've seen are a few mentions in errata and release
notes. setfib(1) doesn't offer much in the way of associated commands,
and definitely doesn't explain how to actually work with a FIB. I'm
curious if there's a command to specifically modify a FIB beyond 0,
besides something like

setfib 1 route add ...

Thanks,
Brent