It is generally straight forward to create a BPEL process (composite) with an existing WSDL. When the wizard prompts to select "synch/asynch" etc, move down the list and select existing WSDL. Notice that JDeveloper generates a wrapper WSDL for the existing WSDL.
If you view the WSDL of your deployed composite, you will see an "import" section in your WSDL that references the real WSDL.
A WSDL generally has one or more bindings defined, and one or more end points (service section) defined. The end points URL is likely not the same as your composite end points URL, since the existing WSDL obviously came from somewhere else. If you leave the endpoints unchanged, then it may potentially leads to big time confusion.
The reason is this. Even though the endpoints are incorrect, you can still deploy your composite, and if you test your composite from the "em" console, it actually works. However, if you try to invoke the composite web service from an external client, such as SOAP UI, a Java client or even from a browser (not the "em" console), then you will see your web service call will fail, because the endpoint URL is incorrect.
There are two ways to deal with this problem.
1) after deploying your composite, you can find out the true URL of your composite from the "em" console. You can update your WSDL with the correct URL.
2) the above solution ends up with hard coded endpoint URL, that's not ideal. The preferred way is to let SOA server decide the URL for you dynamically. To do that, go to the WSDL, comment out (or delete) the entire "Binding" and "Service" section. Now SOA server will treat the existing WSDL just like an abstract WSDL and fill in the binding and service section dynamically for you.