Within the Mechanical GUI the frequency value at which to carry out the harmonic analysis has to be typed in. There is not a way to use the first natural frequency from a modal analysis. However you can potentially do this using APDL command snippets. I am thinking you can do the following:
in the modal analysis in Post1 *GET the first natural frequency. Write this to a file (*CFOPEN, *VWRITE, *CFCLOSE) to a known folder, say, d:\test
In the harmonic analysis read this frequency from the file (*VREAD) and use HROPT to apply the frequency range with NSUB,1.