You can do this using a hidden field and a jQuery plugin called jQuery Mask, which you can get here. Once you have it, put the jquery.mask.min.js file in the following directory on the machine hosting your Forms Server: C:\Program Files\Laserfiche\Laserfiche Forms\Forms\js.

You'll need a single line field for capturing the SSN (class ssn) and another single line field that will be hidden where the value will be stored (class filledField).

$(document).ready(function () {
  
    $.getScript('http://formsServer/Forms/js/jquery.mask.min.js', function () {
      $(".ssn input").mask("999-99-9999");
      $(".ssn input").blur(function () {
        
        if ($(this).val() != '***-**-****') {
        $(".filledField input").val($(this).val());
        $(this).val('***-**-****');
        }
        
      });
    });
});

Replace formsServer with the name of your Forms Server. Then, you can simply hide the filledField with this CSS rule:

.filledField {display:none;}