How to add in a page into a category?

Hi,

I have a drop down menu in my navigation to display the specific categories. However, I also want to add in a page(s) to the same drop down menu. How can I do this?

Below is my code to show the specific categories. I can’t figure out how to go from here.

<?php wp_list_categories(‘include=13,9&title_li=Investing‘); ?>