If there ever ends up being a way and reason for someone to have their own arms, something from a set batch of possible designs would be the way to go for the single-player experience. Modding? That’s up to whatever’s made available.
Otherwise, given the way the game is being made and the way it’s intended to have the story told, I see no way it’d be sensible for people to just be able to make whatever they want. Especially if it could involve uploading something of your own, it’d break immersion quickly.