There are two solutions:
Solder an LED Tactile Button on the Smoothieboard.
Wire a simple button on two pins.
The pause button may be configured as a kill switch instead of pause, this will halt all operations and turn off heaters and pause the host. It can be cleared with a
kill_button_enable true # set to true to enable a kill button
kill_button_pin 2.12 # kill button pin. default is same as pause button 2.12
[kill button]
enable = true # set to true to enable a kill button
pin = 2.12 # kill button pin
toggle_enable = false # if true, button down triggers kill and button up triggers unkill
unkill_enable = true # if true, allows the button to clear halt state (2 second press required when toggle_enable is false)
When in the halted state (after kill button is pressed) the play LED will flash rapidly, whereas when paused it flashes slowly.