I saw a method "[correlateTo(eventId) ⇒ void" for events in the NXSL documentation.
Can I use this method in the Event Processing Policy to correlate events "manually" ? I can imagine a few scenarios for which this would be very useful.
Thanks in advance.

This was intended to be done in Hook::EventProcessor hook script, that is executed before event is processed through EPP rules. But most probably this would also work from a EPP rule. 

This will set "correlated" flag on the event and EPP rules will not react to that event (except for the rules, which have "accept correlated events" checkbox set. 
If you have an alarm originating from the "main" event, then in details of that alarm you should see hierarchy of events.

Thank you, I will experiment with that!