Prasanth,
Attach is the example how you to use to appendToStringList service, also if you would have check the “Built In Services Guide” you would have solve this issue long before so I would recommend to check the documentation of the service before using it.
Also there is no builtin service which converts documentList to stringList, but if you want to do that, you can simply loop over the documentList and append that to stringList like you are doing in the above example but with any branch step.

#Integration-Server-and-ESB#webMethods#Flow-and-Java-services