You don't need a cheat for unlimited staff.
On Steam, right click the game, then go to Properties > Local Files > Browse Local Files
In the new window go to: ProjectHospital_Data\StreamingAssets\Database
Scroll down and look for Tweakables.xml. Open it with Notepad or any other text editor. Change the value of TWEAKABLE_EMPLOYEE_LIMIT to whatever number you'd like your max staff limit to be. I just set it for 500 and it works fine.
There's other things there you can change as well, such as the refund for building costs, max # of ambulances, max # of ambulance calls per day, and even the rate at which your staff improves their skills/levels.
For the skills/levels, SKILL_IMPROVEMENT_RATE_PERCENT and LEVELING_RATE_PERCENT I would not suggest setting it above 75. It seems to bug out if you do... or at least it did for me.