You can also put the U.S. social security number in a column of zero width and fake the formatted value in another cell. Assuming the SSN is entered as a number in B11:
=LEFT(B11,3)&"-"&MID(B11,4,2)&"-"& RIGHT(B11,4)
Instead of the & you can alternatively use the CONCATENATE function on comma separated character strings. It's a work-around until Apple decides to let us make our own formats. Hopefully that will happen soon.