Crafting Spell Damage on Dagger

Where can I craft Spell damage on Dagger, and why Catarina can't do that?

You can't do it at all, probably because dagger are apparently too good. A strange spot to be playing at balance indeed.

So what you need to do is get a dagger that already has spell damage and then craft spell crit or global multi, whichever is missing, those work fine.