Add Watcher to issue using API (XmlRpc or SoapRpc)

We would like to be able to add watchers using the API. Our use case is this: Issues are created by another system, which knows something about the interested parties to the issue.

Currently, we set the reporter to a single person, but we would like to add a couple of other persons to the watch list, and this does not seems possible using the current plugin (http://docs.atlassian.com/software/jira/docs/api/rpc-jira-plugin/latest/index.html?overview-summary.html)