You should be able to do it with the Linkwitz transform in WinISD. I can't find specific documentation on the transfer function of the DCX shelving filters regarding Q0 and Qp, but I suspect they are both 0.707. So in the Linkwitz transform you would enter Q0 = Qp = 0.707. For f0 (the upper corner frequency of the shelving filter), enter the existing -3 dB frequency of the system you're trying to equalize, then adjust fp to a frequency less than f0 to give the desired low-frequency boost. Boost in dB = 20 * log10( f0^2 / fp^2). Then fp should be the new -3dB frequency (now < f0), assuming the Q of the system you're equalizing was 0.707 to begin with.