Let me turn tables: Do you think a web app can be transformed into a web service? If yes, why? If not, why not? Trying to answer these (and posting the answers here), would be a good starting point to know where you might need some help.
Here I have 2 options one is rewrite servlet as a stand alone java class and convert it into webservice other optin is converting the existing servlet application into webservice.
I don't see any real difference between the two. The web service is implemented in its own class, and ususally implements an interface, which is then used to create helper classes. Tools like java2wsdl and wsdl2java (which come with Axis) help in that process. There's really very little code you need to write by hand, except for the service implementation, and possibly some client-side code. I'm sure WL and WS come with comparable tools; they should be mentioned in the documentation.