A few minutes after I post the question I found the answer. Here goes in case anyone needs it (configuration for Mint 18/Ubuntu 16.04):
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 Edit
After upgrading for Mint 19 (homeat home) and Ubuntu 18.04 (workat office) I found the configuration above doesn't work. 18.04 uses a different library for these kind of devices (libinput) and even if I reinstalled Evdev some options don't work. After a painful search I found the solution. Create a a file with .conf extension in /usr/share/X11/xorg.conf.d/ folder. In my case I named it 10-slimblade.conf. Put this configuration inside the file:
Section "InputClass" Identifier "Kensington Kensington Slimblade Trackball" MatchProduct "Kensington Kensington Slimblade Trackball" MatchIsPointer "on" MatchDevicePath "/dev/input/event*" Driver "libinput" Option "ButtonMapping" "1 8 2 4 5 6 7 3 2" Option "ScrollButton" "8" Option "ScrollMethod" "button" Option "MiddleEmulation" "on" EndSection Restart session, and that's it.