kses define: CUSTOM_TAGS ??

In the wpconfig there are definitions that you can set that will never get overridden when WP is upgraded. But if I went in to the kses.php and defined the CUSTOM_TAGS as true it might be set back to false when there is an upgrade.

How would I set this, so that I can use $allowedtags and the other tags defined in there as global variables and set my own rules, so to speak, for what’s allowed and what’s not allowed as an HTML tag?

im assuming this would all be done in the themes functions.php file?