Adding properties to the view model created by using the Knockout JS mapping plugin

You need to use a create method on the mapping object itself like:

var mapping = {
  //customize at the root level.  
  create: function(options) {
     //first map the vm like normal
     var vm = ko.mapping.fromJS(options.data);

     //now manipulate the returned vm in any way that you like
     vm.someProperty = "test";

     vm.someComputed = ko.computed(function() {
          return vm.first() + " " + vm.last();
     });

     //return our vm that has been mapped and tweaked
     return vm;
  }
};