Is it possible to modify the existing out of the box components using SDK ?
You cannot modify the existing ootb components directly with either the SDK or in Studio.
You will have to create your own with FreeMarker. One common way of incorporating custom code into an ootb component, or to replace an ootb component entirely is to use @override.
One caveat is that @override is not currently supported by the SDK, so you will have to contribute that component code in the Components tab in Studio.
If you're new to Lithium, you can find our FreeMarker and FreeMarker context object reference documentation in the Developer Documentation portal. You can also run through an introductory tutorial here and here.