Hardcoding a knowledge ontology that would include e.g. all concepts humans have ever thought of is theoretically possible, since those concepts are made up of a finite amount of complexity. It's just that this would take so very long...

Anyway, I wouldn't rule out that a sufficient knowledge ontology for a FAI could be semi-manually constructed in a century or two, or perhaps a few millenia. It is also theoretically possible that all major players in the world come to an agreement that until then, very strong measures need to be taken to prevent anyone from building anything-that-could-go-UFAI.

I of course wouldn't claim this probability to be particularly high.