Let's say we don't know how he does it exactly, but the ancient kingdom did this kind of stuff, and maybe they used a similar process? It's a matter of extracting the essence of a fruit and then basically transplanting it. If that's the case, and it's what he's doing, then he may have a less complicated process than Vegapunk did.
Honestly, it might be a simpler process than we might think. Especially since it's possible it can happen by accident, if Bunbuku the Tanuki Teapot is any hint.
https://onepiece.fandom.com/wiki/Bunbuku
It's possible that it's as simple as submerging a object in Devil Fruit Juice. "Bathing" a object with juice from a Devil Fruit, in order to give it the power. That's something I could see happening by accident, even though it'd be rare.
And Blackbeard giving Devil Fruits to his guns? Yeah, that's something Blackbeard would do. They'd have to be "throwaway" Devil Fruits, that he didn't want for himself or other members of his crew. But, given how many Devil Fruit Hunts he's had his crew perform, I could see Blackbeard having some "junker" Devil Fruits he doesn't mind giving to his weapons. Good enough to use, but not good enough to give to anyone else.
Plus, they'd probably have to be Zoans, as far as we understand it. Since supposedly objects can only "eat" Zoans. So, maybe he made his own versions of the Ancient Weapons? A bird like a mythical thunderbird or something for Uranos, a elephant-like animal or a mole for Zunesha and Pluton, and some sort of Sea King or turtle for Poseidon as the Zoan Devil Fruits for his guns? Poseidon might be tricky, since any "sea animal" wouldn't be able to actually swim becasue of the Devil Fruit curse. And he couldn't have the base Elephant Fruit, since Funkfreed already has that, but maybe a different model of Devil Fruit would work? But, that could be fun. Blackbeard possibly not just hunting for stand-ins for the God Fruits for himself, but also Devil Fruits that "mimic" the power of the Ancient Weapons, too.
So, yeah, there's potential here. I could see it.
