Is it possible to re-use headers in Paw?

The best way to do so is to have an environment domain for that. Make one variable per header, you may have a variable called Content-Type (as an example) and give it the application/json value for one environment, or application/xml for another environment. This way you'll be able to switch between JSON and XML in many requests at once.

In the example above, there are 2 variables set, one for the Content-Type and another one for Accept. Both variable are then used in the request. As you can see in the preview at the bottom, the variables are replaced by their values in the HTTP request.