Loc: Michigan USA
Depending on what you consider retrofitó It is permitted to protect the circuit with a Ground Fault Circuit Interrupter type circuit breaker and this would allow you to install a 3 wire receptacle (2 wire w/ ground) on the circuit and this would be code compliant, if that's what you were interested in. Beyond that, I agree with Greg.
Usually when I hear this question it is because the customer wants to install something that needs a ground. Most stuff in a home has a 2 prong plug so it is not an issue but when you are installing computers that need surge/EMI protection or equipment with grounded cases there is nothing better than a real hard wire ground ... even if the code allows GFCIs. Bear in mind, in most cases the fault will go through the user to operate the GFCI. I hope they know to test them often.
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