A few minutes after I post the question I found the answer. Here goes in case anyone needs it:

 xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Middle Button Emulation" 8 0
 7 8 9 
 xinput set-button-map "Kensington Kensington Slimblade Trackball" 1 8 2 4 5 6 7 3 2
 xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation" 8 1
 xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Button" 8 8
 xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Axes" 8 6 7 4 5
 xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Timeout" 16 300