A USB cable is just four wires so in simple terms to make it removable all you need to open the case, cut the cable slightly longer than you need (just in case) and solder on a socket with the wires
in the right order. There are many harder things to solder, but a little experience would be helpful. Either put the socket in a
helping hand while you solder it or if you don't have one put it on the end of a cable so you have something to grip.
The hardest part is enlarging the hole in the case to fit the socket into without it looking bad, and keeping the socket in place in said hole as it is repeatedly pushed inwards whenever you plug in a cable.
Hard is relative, I'm assuming there is a bit of space in the case as if there isn't this might be much harder - open it up and have a look
