well.. if you can't replace the pushbutton with a 3-pos toggle then I would suggest writting some code to count the number of presses.. sorta a morse code for the PLC to know what to do. Of course this can be a pain do to scan times, etc... missing scans or pushes so you would need some kinda feedback such as an LED to let you know that the plc has indeed counted the 'push' of the button.
1 push = auto
2 pushes = on
3 pushes = off
simple
