How can i achieve something like this

public class Test
{
  public string Name{get;set;}
}

//And i want to do
Test.Add(new string Surname)

//Result 
public class Test
{
  public string Name{get;set;}
  public string Surname{get;set;}
}

public class Test
{
  public string Name{get;set;}
}

//And i want to do
Test.Add(new string Surname)

//Result 
public class Test
{
  public string Name{get;set;}
  public string Surname{get;set;}
}