I don't know what the load sheet looks like, but I guess you could get the arms of the various stations by dividing the x axis through the y axis (or the other way around). Or something like that.
Just remember: moment = mass times arm. So mass = moment / arm, or arm = moment / mass. Your load sheet should have two of the three parameters, so the third one is easy to calculate.
What aircraft are you talking about? Although the empty mass and arm will vary amongst planes, the datum of the plane and the arms of the various stations will most likely not. Perhaps someone on this forum has the data for your type of plane already?