Hi Rahul,
I'm not sure exactly what you are asking, but my gut tells me it is possible. You may want to check out the demo phone (
http://sipjs.com/demo-phone) for a simple example of a SIP.js client.
SIP.js is structured in a way that separates the signaling from the media, so there is a lot of flexibility for compatibility between different media endpoints. In the default case, it uses WebRTC to generate its SDP. If you have any other backend that generates SDP (such as another WebRTC service, with its own signaling system), you can create your own MediaHandlerFactory. Just give SIP.js the SDP and it should be compatible. It won't be easy, but it is definitely a possibility.
Cheers,
Will