You do not need the ImpactJS engine, it's a completely different engine to C2 and will require you program your game.
I have been planning to make a tutorial, but it is simple to get going, basically:
1. Open the appMobi XDK.
2. Create a new project.
3. Export your C2 game with the 'export for appMobi' option. Export over the appMobi project you just made. Usually its path is something like Documents\AppMobi XDK\MyProject\3.4.04. Click the refresh button in the XDK
5. You should now see your game running - you can try testing using 'Test over Wifi' or 'Test Anywhere', and also build for the app store.