Ok thanks, I made it but I am wondering if I can easly change keycaptcha type (from default to M+ KeyCaptcha).
Another thing is :
I added keycaptcha to thread_view template in "quick reply" form.
It works OK but when I click "more options" button the Captcha checks value... It is unnecessary but I am afraid that to stop this process I have to modify KeyCaptcha javascript files :/