If you're asking can they physically digest it - sure they can. When I had cats - Patches and Ginger - they used to eat my veggie burgers. I'd crumble up the veggie burgers in their dish and they'd lick it clean. I think they liked the veggie burgers better than their cat food.
Now if you're asking will cats like a hard-boiled egg - well that's definitely a question for them :) But if they're not used to eating it, then you need to introduce it slowly. And checking with the vet might get you started.