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 <config> node.

<frontend> <routers> <checkout> <args> <modules> <Your_ModuleNameHere before="Community_ModuleNameHere">Your_ModuleNameHere</Your_ModuleNameHere> </modules> </args> </checkout> </routers> </frontend> 

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 <from> and <to> tags then it won't work.
(and I have no idea on how to override that).

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 <config> node.

<frontend> <routers> <checkout> <args> <modules> <Your_ModuleNameHere before="Community_ModuleNameHere">Your_ModuleNameHere</Your_ModuleNameHere> </modules> </args> </checkout> </routers> </frontend> 

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 <from> and <to> tags then it won't work.
(and I have no idea on how to override that).

[EDIT]
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.