using System;

using AutoMapper;

public class Program

{

    public static void Main()

    {

        Mapper.CreateMap<Foo, Bar>()

        .ForMember(dest => dest.Notes, opt => opt.Ignore()) 

        .AfterMap((src, dest) => dest.Notes = string.Concat(dest.Notes, src.Notes));

        var source = new Foo { Notes = "A note" };

        var destination = new Bar { Notes = "B note" };

        Mapper.Map<Foo, Bar>(source, destination);

        Console.WriteLine(destination.Notes);

    }

    public class Foo

    {

        public string Notes { get; set; }   

    }

    public class Bar

    {

        public string Notes { get; set; }   

    }

}