Of course you can.
You could also stick your finger in a socket and jump off the roof headfirst.
Anything is possible.
Will it work? That’s a completely different story.
You can convert from TN-C to TN-C-S, but you have to do it properly and sensibly. There are regulations and guidelines for this.
Without knowing the installation, the ground conditions, or what knowledge you have of what you want to do, we can’t say anything specific. Of course, you must be aware that simply driving a rod into the ground does not guarantee that the installation will be properly earthed, nor that the RCD will provide protection when needed.
Earthing is not just a matter of driving in a pin or two.
Once the earthing has been carried out, measurements should be taken to assess the results and check whether they meet the PPOR protection requirements for the installation in question.
In many types of soil, earthing may be ineffective and require more work and resources.
Similarly, the separation of the PEN conductor into PE and N is subject to regulations and has its own requirements that must be met.
If you have any doubts, hire a professional who will also provide you with the relevant reports certifying the effectiveness of the earthing.
