This may depend on how the extension is overriding the controller but here goes. You need to add in your module's
config.xml file, this as a child of the
If the community module overrides the cart controller in the same way as described above then it works. If it uses the 'old method' with
<to> tags then it won't work.
(and I have no idea on how to override that).
Make sure that your controller is also named
CartController and it's located in
Namespace/Module/controllers/CartController.php. Also you might want to make your local module depend on the community module but I don't think this is mandatory. Also make your controller class extend the community module controller.