We need, first, to realize that headers are not headings.
What is the difference between headings and headers?


I am assuming you are talking about Heading styles.


You can use the Organizer to copy styles from your template to existing documents. If you want all of your styles in your normal.dotm template to transfer it can be done with a simple macro.
See:

• Macro to copy styles from one template to another - Microsoft Community
• Update Styles Options

See also: Why Use Word's Built-In Heading Styles? by Shauna Kelly