The concept "Phase of the THD" does not exist but if you are interested in the relative phase of the different harmonic components with respect to the fundamental it can be done.
I'll suggest you to copy and modify the "Harmonic distortion Analyzer.vi" that is located in your Analyze-->>Waveform Measurements palette.
The VI itself only exports the amplitudes of the different harmonics, but internally you can access the phase information from the 2 sub-VIs called "Extract Single Tone Information from Hann Spectrum.vi", one VI giving you the phase of the fundamental, the other one the phases of the different harmonics.
If Phi_1 is the phase of your fundamental and Phi_n the phase of your n'th harmonic, the relative phase can be computed as:
PhiR
elative_n = Phi_n - n * Phi_1
