Someone will surely correct me if I'm wrong on this but I believe with Event Manager you cannot do this because Event Manager looks for matching entries in the ev_events table to update an incident it has raised, which is why after the housekeeping period it cleans up these processed records and any subsequent entries that would've matched previously would raise a new incident.
MAP or ALM should be able to handle this though, as they can generate an external action from a manual incident then report back to servicedesk on completion. They use a similar interface in servicedesk as event manager but with a wider array of parameters and some external configuration required.
Hi John. Hadyn,
This is possible by calling the PostIntegrationEvent action of the Event Manager web service rather than PostEvent or SendEvent. However the application calling it needs to know the Guid of the incident to update (something that the round trip automation knows about because it is included with the outgoing event). At the moment there is no way to do it using just the reference number.