can a web application converted to webservice?

I know how to convert standard java application to webservice. But is it possible to convert a web application to web service? how?

how many types of WSDL's are there?

Neither a desktop application nor a web application can just be "converted to a web application" in a meaningful, general way. What you have done is probably extract core functionality from the app, and made those available via web service calls. You can do that for web apps as well by replacing calls from the presentation/view layer to the business layer with web service calls.

I'm not sure what WSDLs have to do with this. Whether the web service is described by one WSDL or multiple is not related to the question above.

For a more precise answer we'd need to know what you're trying to do in more detail.