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The prestress from the static analysis has the effect of stiffening (or weakening, as the case may be) the structure only. The static stress is used to calculate an additional stiffening matrix, which in addition to the regular stiffness matrix gives the total stiffness (paraphrased from Theory Reference 3.4). The prestress will, therefore, directly affect the stiffness of the system and potentially affect the modal/harmonic stress, indirectly. In other words, the prestress is not added to the modal/harmonic results.
If you would like to postprocess the combined stress from the static and harmonic analysis, you will need to create a load case combination; for example, consider a cantilevered beam with a static preload and subsequent harmonic load at the outer end. The stress from the preload represents the mean stress in the combined analysis, and the harmonic stress represents the deviatoric stress. Since the harmonic analysis yeilds zero-based stress results, the total stress will be the sum of the preload stress and the harmonic stress.
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