There is a plugin that appears to do what you want: https://github.com/NoahY/q2a-admin-plus
But it's not maintained, and I wouldn't recommend doing that anyway.
The best way to run custom PHP code is to make a plugin. Depending on what you want to do, the best type might be a page plugin, widget plugin, or a process plugin (init_page function). See here for more: http://docs.question2answer.org/plugins/