Can't say that I've ever checked, but I imagine it would be a little tricky to do. My refractometer only goes up to 36 BRIX, and I imagine it's a lot higher than that, and probably to viscous to use your hydrometer, and it's probably off of that scale too. You'd probably have to use a known quantity of fpack and water to thin it out to get a reading, and then convert it back.
Based on my past experience, the fpack typically raises the SG of 6 gallons of must around SG .015. Working backwards from there on Fermcalc, using starting SG of 1.090, and final SG of 1.105 (an increase of .015), for a 6 gallon must, it would take 2.4 pounds of sugar. If (I repeat IF) the fpack is 1 quart in total liquid (ignoring skins, seeds, etc), the BRIX of that liquid would be 81, SG would be 1.421. Purely hypothetical based upon the volume.