Is there a way to save the existing header menu as a template and then duplicate it?
If you have a header from the Theme options, there is no option to convert it to Header Builder.
If you already have a header template made with Header Builder, you can use YOAST Duplicate Post to duplicate it. Notice that with this plugin you need to activate the appropriate post type in this plugin settings.
Thanks Phil. It seems like when I want to make a new header template, it blows out the theme header and I don't know how to replicate the exact same settings from the theme header.
I suggest checking the following video tutorials:
There you will find out how to use the Header Builder.