there are plenty of mods with secondary fire via scripting, a quick google search in this site will return tons of results.
Anyway, take a look here, you can see how to get your new alt fire button via scripting, http://www.doom3world.org/phpbb2/viewtopic.php?p=235993#p235993
Then it's just a matter of making the thing you want when the button is detected.
Oh and keep in mind that you have to bind your new button to a key, e.g. you could put something like this in the autoexec.cfg,
bind "MOUSE2" "_button5"
Vanilla D3 actually has 3 unused button ( "_button5, "_button6" and "_button7" ) which can be detected via scripting by using the approach you see in the post I linked, you don't need the SDK at all.
Also, Ivan_the_B found a good trick to gain access to unlimited keys, http://www.doom3world.org/phpbb2/viewtopic.php?p=218719#p218719
Hope this help you, BTW I think I've already made something like that ( the 3 rounds burst ), let me know if you need the code