Yes, It should be possible to create JSB for Facebook Connect, Game Center, etc.
The script that generates the bindings is generic enough to support "any" Objective-C/C projects... ( For the moment it parsed cocos2d, Cocosbuilder reader, cocosdenshion and Chipmunk without problems. )
the instructions to generate bindings are here:
@jason / noocell:
It is planned to add that feature. For the moment what you have to do is something similar to what I am doing for the cocos2d-html5 tests.
1. Place all your JS files in one directory
2. Place all your resources in another directory
3. Include them in your Xcode project and in your "HTML5 project"
3. Create a "bootstrap" for JSB
4. Create a "bootstrap" for HTML5
5. Run them.
As an example, download cocos2d-html5 from github and cocos2d develop-v2 branch from github too, including its submodules and you will see that the code and resources are shared.
bootstrap for JSB:
bootstrap for HTML5: