Ladies and genttlemen... it's solved.
Here is what I did:
1) XFree86 must think mouse does have just one button (it's because I do really want to avoid any button but the first):
Option "Protocol" "auto"
Option "Device" "/dev/mouse"
Option "Buttons" "1"
The lines above go inside /etc/X11/XF86Config-4.
2) xmodmap makes the left mouse button be the sixth button, acording to imwheel manpage this is the thumb left button (this line goes inside $HOME/.xinitrc):
xmodmap -e "pointer = 6"
3) imwheel acts over the new sixth mouse button (this line goes inside $HOME/.imwheelrc):
None, Left, Button1
Shift_L, Left, M
Shift_R, Left, M
A little explanation... the first line makes imwheel act over ALL windows; the second tells to imwheel that the sixth button is infact the first and the 2 last tell to imwheel that when someone press Shift (Left or Right) + the sixth button it have to translation into an M (it could be any keysym).
I wont write too much because certainly I'll make too much mistakes, but to conclude... the idea is make the left button be another button whose can be controlled by imwheel. Furthermore, this solution is intended to be used in an implementation of a kiosk, that's why I made a keysym combination be translated into another one, that's why the mouse just does possess one button, etc.