You can use Borax in soap for use on people. As soapmedic said, it's commonly used in liquid soaps. It works great to help thicken it.
I disagree that adding Borax will nuetralize hydroxides, it may buffer the PH a tad but that's it. Both it and hydroxides are alkali after all.
A good reason to add borax to CP soap would be to help it clean better and to for lather. Borax doesn't cause soap to lather any thicker, but it does cause it to release it's lather more easily.