How to add field to ALL webforms?
You have 2 options:


Go to /admin/structure/types/manage/webform/fields and add a field. Then all your new webforms will use the new field.
Install the Webform Share module, create a webform as usual, create your field, click the Export tab, go to /admin/structure/types/manage/webform and paste your configuration in the Publishing options &gt; Webform default components box.


If all you need is one simple field, I recommend the former option.