Sidebar specific to single page

I am using catchbase pro with default layout Three columns (secondary sidebar - content - primary sidebar). I wish to insert a different sidebar in place of the secondary sidebar on one page on my site, but the optional page sidebar and the 3 optional sidebars insert themselves into the primary place. How can I make the special sidebar appear in the secondary's place? Also, may I know why there is no layout option of Three columns (primary sidebar, content, secondary sidebar)? This seems to be the most logical default. Many thanks! Ripper