Extending bbcode tag set

I'd like to add a couple of bbcodes, one for preformatted text (I know it can play havoc with layout) and eventually when its needed for definition style lists (dl/dd/dt),

The pre tag is the one I want to add first, but looking at the punbb_bbcode extension module I can't see where bbcode tags get expanded out to their html equivalents...

So if anyone has a guide or a set of steps I need to follow to add custom bb tags would be grateful.

Might just take me a few goes as used to all my ws stuff in perl rather than php but finally having to give in to php...

edit1. changed subject.