Have you satisfied the unlock condition stated on Employees (Game Dev Story) to unlock Chimpman Z. Force? And I think you may misunderstand how getting a new career works. You need to get a single employee up to level 5 on the director & producer careers (using career change manuals) and then you'll have the option to change that employee's career to hardware engineer. More on careers at: Careers (Game Dev Story) (specifically the "Changing Careers" section).
To have Hardware Engineer you need to have level 5 on all the jobs of one emplyee. The quickest way to get the Hardware engineer at the cheapest rate is to upgrade Newb Ownerton who you have at the start if you are quick you will have Hardware Engineer by Y9-Y10. Hope this helped