A Stacklands mod that stacks cards to the bottom of another stack rather than to the top when stacking on top is not possible.
This is mainly useful to add cards to stacks that are being worked on by a Villager, e.g. Apple Trees and Berry Bushes.
Note this will never create an illegal stack of cards. Stacking X to the bottom of Y is always possible manually in the base game by dragging Y on top of X instead.
This mod simply makes the process more convenient when you start with X you do not need to put it down, grab Y and drag it on top of X but instead just stacks X to the bottom of Y when you drop it on top of Y.