I put a micro USB port in mine over a year ago. I also created a cable for it which contains a USB SD card reader. It's not hard at all if you can find an SD card reader that is compatible. This is the exact one I used, but there are definitely some others that are known to work.
There's a list here where it says "Only the following SD card readers work with the New 3DS XL". It's not all-inclusive and not very big, so if you already own some, try what you have and it may work.
Ok I just got in the anker sd reader in and my laptop doesn't detect my nand and my error code on the N3Dsxl is 00000400 when I put it in the ad reader and my operating system is windows 10
I'm not sure what to make of the error code because I know sometimes mine produces different error codes while it still works the same. Make sure your wires aren't too long. You may have to connect things in a certain order to make it work. This is what I have to do: