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If you are not using the PSD for a fatigue analysis (the more normal case) and simply want to do an allowable stress evaluation then I think it is reasonable to add the static stresses to the 3 sigma PSD stresses. There is still the issue that negative static stresses will cancel out the unsigned 3 sigma stresses. If there was a way to combine the absolute value of the static stresses with the 3 sigma stresses that would be more correct. That's a feature of APDL POST1 load case combinations. Some industries do treat the 3 sigma stress as an upper bound (which technically it isn't.)