Hello Ashley,
If you create a custom header, you can insert several Bar elements and add any elements you want to display. You can then assign the header to a specific page or a group of pages. What is not allowed is assigning two custom headers to a specific page or group of pages.
Anyways, you can use the Dropdown element if you want to have a dropdown in your custom header. You can simply add it next to the current header items.
Best Regards.