To hook into a third-party service - you write an CQ OSGi bundle that contains operations that can invoke the third party service. In your custom OSGI service -- you can wrap a Java API that knows how to invoke operations of the third-party service.
We have community articles that talks about hooking into Google Restful services (as an example of hooking into a third-party service). See:
http://helpx.adobe.com/experience-manager/using/restful-services.html
Likewise -- if the 3rd party service exposes a WSDL - you can write an OSGi bundle that contains Java proxy classes that invoke operations exposed by the WSDL SOAP stack. See this article:
http://helpx.adobe.com/experience-manager/using/creating-cq-bundles-consume-web.html
Another example of hooking into a 3rd party service using web services is an article that hooking in Adobe LiveCycle ES into CQ. See:
http://helpx.adobe.com/experience-manager/using/integrating-livecycle-cq-applications.html
Hope this helps.
