There are a ton of items on PC that the player cannot get, you can forcibly add them to your inventory but they don't show. You can even equip them with console commands, like the black mages hood... I love that thing. I'm fairly certain her hood is going to be just like that. It'll take a group of modders to unlock it.
Not with the creation kit. And Serana was created as any other character was. The used the "female" skeleton layout (of course) for her, but the items are designed for both, unless the hood is a part of her skeleton, which i highly doubt, as they would need to create a whole new character system from scratch.