How to add getter to an existing object

I can have a getter in a JavaScript object like this:

var member = {
    firstName:"XYZ", 
    lastName:"zzz", 
    get fullName(){ return (this.firstName + ' ' + this.lastName); }
}

I can even add more properties on the fly, like this:

member.isGuest = true;

But, is there any way we can add getters to an existing object? Something like this:

member.isGuest = get isGuest(){ return this.firstName == 'Guest'; }

Answer

try defineProperty

Object.defineProperty(member, 'isGuest', {
  get: function() { return this.firstName=='Guest' }
});