Try something like this in the do_markup() function, located in ubbt.inc.php
$Body =preg_replace("/\[yourtag\]/i","<span class="yourclass">",$Body);
$Body =preg_replace("/\[\/yourtag\]/i","</span>",$Body);
and in undo_markup():
$Body = preg_replace("/<span class="yourclass">/","[yourtag]",$Body);
$Body = str_replace("</span>","[/yourtag]",$Body);
Replace "yourtag" with the keyword, you want to use.
Replace "yourclass" with the css classname, you want to use.