While you probably wouldn't want to do this yourself (it certainly doesn't look comfortable!), one Japanese gamer apparently forced the recently released 3DS XL on the Circle Pad Pro and got it to recognise the larger 3DS — which makes sense!
Nintendo will release a Circle Pad Pro for the larger 3DS. If you don't like destroying game peripherals, it's probably best to wait for that.
[3DS]3DS LLに拡張スライドパッドを無理矢理付けるとそのまま認識するらしい [■■速報＠保管庫(Alt)■■]