drawing using mouse and key button

In some 3D application, I use all the 3 mouse buttons. I would like to activate strokeIT pressing a key (like CTRL for example) and after draw something whit my mouse. Now in option/general/draw_using I have this choice:

left mouse button
right mouse button
center mouse button

I need a key to combine whit mouse button, like:

left mouse button + CTRL
or
right mouse button + ALT

whatever you want (better if I can decide which key button I want to combine whit mouse)

sorry my bad english

mitja