This is effectively a support issue, that is currently unsupported, which I mentioned in the Support section. Essentially I think there should be a way to easily (!) open as many folder and subfolders into one contact sheet as one likes to. Whether that be by dragging the folder (or Option, Command etc drag) or other method doesnt really matter, as long as it can be achieved.
The the contact sheet can handle thousands of images from several folders we should be able to choose these folders easily - perhaps by simply using 'Shift' or 'Command' to add to a selection of folders (and their subfolders) that we lie to have opened in one contact sheet.
At present we are limited to the first selection of images from one folder (and their subfolders). Any subsequent folders one likes to add, only give the option to replace or open in a new sheet:
