More widget zones

Is it possible to add more widget zones to the dropdown in the admin section? I would like to add a "Recently Viewed" JCarousel to the widget zone  "order_summary_content_after" to display in the cart page

Added the zone to: AdditionalSupportedWidgetZones.xml seems to work :)

Yes, that will work.
Actually you can also use SupportedWidgetZones.xml file.
The idea of the SupportedWidgetZones.xml file is to contain all the common widget zones, while the AdditionalSupportedWidgetZones.xml contains all the theme specific widget zones.
This way each theme can have additional widget zones.

Thanks