People differ in flexibility.
When I do my Pilates exercises on a regular basis I am in need of a much shorter stem.
The length of my back is not just the difference between leg length and body length.
My question: How can I adjust the bend of the riders back, in Fit Advisor? 
The fit advisor can be customized to collect any combination of body measurements you like. For more see, bikecad.ca/customizing_fit_advisor and bikecad.ca/customizing_fit_formulas.
Currently, the torso is approximated by a straight line connecting the pelvis to the base of the neck. If the back is more bent, you can account for that with a shorter torso length.
The purpose of the fit advisor is to recommend frame geometry based on a rider's body measurements. If you would prefer to visually represent a rider on the screen and apply your own judgements about frame modification based on your own manipulation of that model of the rider, you may prefer to enter body measurements into the more comprehensive rider anatomy tab in the fit advisor. This is described at: bikecad.ca/rider_anatomy_tabs.
You can also use the Fit advisor to approximate values for these more comprehensive dimensions. It sounds like you might like to control how these approximations are made. This is described at: bikecad.ca/customizing_fit_advisor_2_rider_anatomy.
When doing all this, you may also like to import a photo of one of your subjects. This is described at: bikecad.ca/import_photo.