public class MyTypeConverter : TypeConverter<String,String>
    {
        public MyTypeConverter(IMyService service)
        {
            MyService = service;
        }

        public IMyService MyService { get; set; }

        protected override string  ConvertCore(string source)
        {
            //use service
        }
     }

用法：

Mapper.CreateMap<string,string>()
                     .ConvertUsing(new MyTypeConverter(_myService));