From the BPMN2 specification: "A Service Task is a Task that uses some sort of service, which could be a Web service or an automated application.". So you should use a serviceTask in that case. On the contrary, the "task" don't model a specific semantic (tasks are "generic" activities).
One thing: you probably cannot "convert" a task to a Service Task: if you don't add an interface as parameter, it will not work! (I just opened a thread in the dev forum about that: http://community.jboss.org/thread/173063?tstart=0 )