Assuming that, the column JoinDateQuery will only have two values i.e. "(select hiredate from employeeInfo)" or "(select getdate)" then you can define a bit column HasHireDate instead of JoinDateQuery, with this you can define a simple static query like this....

SELECT H.EmpID, COALESCE( E.HireDate, GETDATE() ) FROM NewHireEmployee H LEFT JOIN EmployeeInfo E ON H.EmpID = E.EmpID AND H.HasHireDate = 1

WHERE H.EmpID = @iEmpID