public class Src
{
    public  MyDateTime DateTime { get; set; }
}

public class DstBase
{
    public DateTime? DateTime { get; set; }
}

public class Dst:DstBase
{
    public new MyDateTime DateTime { get; set; }
}

            config.ForType<Src, Dst>()
                .GenerateMapper(MapType.Map);

        public static MapsterBug.Dst AdaptToDst(this MapsterBug.Src p1)
        {
            return p1 == null ? null : new MapsterBug.Dst()
            {
                DateTime = p1.DateTime == null ? null : new MapsterBug.MyDateTime(p1.DateTime.DateTime),
                DateTime = (System.DateTime?)p1.DateTime.DateTime.Value
            };
        }
    }