SqlQuery into a [NotMapped] field?

I had the same trouble using stored procedures to do selects with calculated fields. I created a view model that looks exactly like my entity without the db annotations. Then after I call my stored procedure using the view model I select into my entity. So, using your example above:

public class EmployeeVM
{
    public int EmployeeId { get; set; }
    public string EmployeeName { get; set; }
    public string CustomerName { get; set; }
}

Then you can call:

public List<Employee> GetEmployees()
{
    using (MyContext context = new MyContext())
    {
        return context.Database.SqlQuery<EmployeeVM>("select E.EmployeeId, E.EmployeeName,
 C.CustomerName from Employee E left join Customer C on E.CustomerId = C.CustomerId")
    .Select(x=> new Employee(){
        EmployeeId = x.EmployeeId,
        EmployeeName = x.EmployeeName,
        CustomerName = x.CustomerName
        }).ToList();
    }
}

Hope this helps.