WYSIWYG Plugin for templates.php

I use the WYSIWYG Plugin (http://www.moiblog.com/plugins/) in my WP blog to make posts and pages fast and easy. But because javascript and php don´t work with my WP pages (believe me, I use Runphp plugin, and check the eval() Content option, but there´s no way), I need to use external files for some functions, and I want to edit them in a easy way without seeing only code. That´s my question: there´s a way to put the WYSIWYG plugin on the edit area of wp-admin/templates.php to modify external files?