Hi
Please confirm that you talk about current harmonics (and not voltage harmonics).
(The difference is, that with "current" you simply can focus on your DUT, while with voltage you need to do two measurements with and without your load (to see how much is caused by your DUT) and you need to take mains impedance into account)
You surely can do with a digital system like picoscope ... but the results will deoend a lot on:
* used signal voltage range
* sample rate
* anti aliasing filter
* windowing function (before performing the FFT, because you can't be sure that the sampling frequenc is an integer multiple of mains frequency)
* window size
* mains frequency stability
* mains waveform stability
I agree with FvM, that a refernce measurement with known overtones (but all others signal values should be about identical to your real test signal) is useful to determine accuracy and precision of your test setup.
Klaus