In case of spine injury, some doctors are trying to use some of the nerves of the arms to partially regain control of the legs. The operation has been carried out on some patients, and it does seem to work, although there is a (re)learning phase and the control is very superficial.
Just like a child learning how to use its body, the patient needs to learn how to use those redirected nerves.
We can use this ability to add new features to our bodies.
Imagine adding a sonar to a blind person. The sonar's outputs don't even have to be linked 'directly' to the nerves, they could actually use some surface of the skin where there are pressure and temperature sensing nerves.
We could add the equivalent of a third eye that way. Or any other device actually, although the amount of skin usable might run out :)
The next step is of course to add some of the devices inputs and outputs directly at the neural level...