In this case you must use aliases. Approx code:
var employeeQuery = qf.Create()
.From(qf.Employee
.LeftJoin(EmployeeEntity.Relations.EmployeeEntityUsingManagerId, "emp", "man"))
.Select(() => new RptEmployee()
{
Id = EmployeeFields.Id.Source("emp").ToValue<int>(),
EmployeeName = EmployeeFields.Name.Source("emp").As("EmployeeName").ToValue<string>(),
ManagerName = EmployeeFields.Name.Source("man").As("ManagerName").ToValue<string>()
});