couple different things you can do, first basic structure:
The file default.asp in the main /forum directory has an include statement to pull in the file /includes/header.asp
You can add html (tables, div's, etc) at the end of the header.asp file and they will be included at the top of ALL of your forum pages.
So, editing header.asp will affect all of your pages.
trickier if you just want to add something for individual pages. You can modify default.asp, look around line 388 where the header file is included, right after this, and before the main table is called you can add html that will appear on all forum pages.
MAKE A COPY OF ANY FILE YOU EDIT BEFORE YOU START!
Not sure what editor you're using, but you're going to have to do all of this is code view, the way the webwiz code is parsed it does not preview perfectly before publishing.
The sky is the limit to what you add, my forum is at
www.swvhc.com/forum, I'm running three columns, separate headers and footers. Just got it up and running, took a while to get the mods right. I'm writing in pure CSS, no tables for layout.