Yes, I can use the CPanel 'Custom Error Pages' section to do that. All of my error pages are currently set up this way, but I've run into a problem: These pages are shown incorrectly when a user requests a missing file from a subfolder, not from the root folder of the site -- pictures and CSS aren't visible, and the page looks horrible. This happens because all paths to pictures and CSSs in my 404.shtml are relative, not absolute.
I don't want to use absolute paths to pictures in the 404.shtml, and would prefer to simply redirect it to another page, say 404err.htm, which could be placed in the root site folder.
Currently, I'm redirecting from their default 404.shtml to my own error page using the meta refresh tag with no timeout, but I think there should be a way to do this from the server side (using .htaccess) -- it should work faster than the meta-refresh.
Please, please help me find that .htaccess file!