I think it could be done quite easily, replicating existing codes. You will need to draw the button on your own though.
So, for example, take a look in templates/main_post_form.html and replicate the code from let's say the bold tag this way:
Put and translate $l_bb_strike into your language pack (lang/eng.php probably).
Then modify bb_codes.php, replicating for the function enCodeBB:
and for the function deCodeBB:
Don't forget about the button, which should be placed under /img/button_strike.gif
That would be all...