Mapper.CreateMap<Employee, EmployeeDto>()
  .ForMember(d => d.FullName, opt => opt.MapFrom(src => src.FirstName + " " + src.LastName));

var employees = await db.Employees.ProjectTo<EmployeeDto>().ToListAsync();

public class Employee {
  [Computed]
  public string FullName { get { return FirstName + " " + LastName; } }
}
Mapper.CreateMap<Employee, EmployeeDto>();

var employees = await db.Employees.ProjectToListAsync<EmployeeDto>();