MapFromReverseMap.cs

static void Main()
{
    Mapper.Initialize(cfg =>
    {
		//cfg.AllowNullDestinationValues.Dump();
		cfg.CreateMap<TimesheetModel, TimesheetViewModel>()
			.ForMember(d=>d.Contact, o=>o.MapFrom(s=>s.ContactNavigation.Id))
			.ReverseMap();
    });
	Mapper.AssertConfigurationIsValid();
	try{
		Mapper.Map<TimesheetModel>(new TimesheetViewModel()).Dump();
	}catch(Exception ex){
		ex.ToString().Dump(); 
	}
}

public partial class TimesheetModel
{
        public  int ID { get; set; }
        public  DateTime? StartDate { get; set; }
        public  int? Contact { get; set; }
        public  ContactModel ContactNavigation { get; set; }
}

public class TimesheetViewModel
{
        public int? Contact { get; set; }
        public DateTime? StartDate { get; set; } 
}

public class ContactModel
{
	public int Id { get; set; }
}