Hi Roshan Agrawal,
I think the answer for your question is yes, jbpm engine should be able to be used as a service.
If you try the demo from jbpm5.3 or jbpm5.4 final full installer, you will see that there is a jbpm-console which is a standalone application that provides REST API for other application to interact with the JBPM Process and Human Task.
Just by looking at this, it tells us that we can use the JBPM as a service.
So the idea is, you can make 1 application that embed the jbpm core engine, then make this application expose its service interfaces. Your other applications just need to talk / call this application's service to interact with the process.