Brain plasticity to the rescue. There are examples of people being able to integrate completely new senses (vibrating compass belt) and controlling prosthesis with completely unrelated nerves.
But if you want to graft additional limbs to people, I highly recommend starting with baby aged humans already, their brain plasticity is unmatched. Imagine how much more fulfilment an amazon fulfilment center worker could bring with four arms! I should get Bezos on the phone.