Since you can't get at it directly, you might have to do it by hand. I've played with that when trying to integrate my sub while the mains are on (a different approach than measuring the sub by itself and later integrating mains and sub).
I would suggest to make a house curve that meets your needs, then load it into REW. Then iteratively apply and tweak filters until you hit that curve. Last time I went around with this, I just ignored the REW predictions and set all my own center frequencies, gains and bandwidths.