Dynamically add properties to a existing object

It's not possible with a "normal" object, but you can do it with an ExpandoObject and the dynamic keyword:

dynamic person = new ExpandoObject();
person.FirstName = "Sam";
person.LastName = "Lewis";
person.Age = 42;
person.Foo = "Bar";
...

If you try to assign a property that doesn't exist, it is added to the object. If you try to read a property that doesn't exist, it will raise an exception. So it's roughly the same behavior as a dictionary (and ExpandoObject actually implements IDictionary<string, object>)

Take a look at the ExpandoObject.

For example:

dynamic person = new ExpandoObject();
person.Name = "Mr bar";
person.Sex = "No Thanks";
person.Age = 123;

Additional reading here.

If you can't use the dynamic type with ExpandoObject, then you could use a 'Property Bag' mechanism, where, using a dictionary (or some other key / value collection type) you store string key's that name the properties and values of the required type.

See here for an example implementation.