You can't just add the editable region to the template because the theme page is now separate and the pages are based on the theme. If you add the same named editable region to the theme, then it will be added to all of your pages and the content can be edited.
In my tests I created a base page with one editable region in the main body area.
Turned that to a template and then applied it to my first test page (page1.php)
Next I opened the DWT and turned that into a theme.
Then created page2.php as a "Create Page with Theme"
Next I decided I needed a new editable region since there is some client change etc that will require a new area for the whole site.
Open theme, add region and save.
Now all "new" pages created with the Theme show the new editable region. But all pages prior do not.
Prior pages do show the region with default content when rendered in the browser but they not display this in the page so you can not add content to the new region.
I'll post a couple screen shots as an example: