You can’t do exactly what you want, but you can get pretty close by specifying mapping options when you call Map. Ignore the property in your config:

cfg.CreateMap<Message, MessageDto>()
    .ForMember(dest => dest.Timestamp, opt => opt.Ignore());

Then pass in options when you call your map:

int someValue = 5;
var dto = Mapper.Map<Message, MessageDto>(message, opt => 
    opt.AfterMap((src, dest) => dest.TimeStamp = src.SendTime.AddMinutes(someValue)));

Note that you need to use the Mapper.Map<TSrc, TDest> overload to use this syntax.