What if you have several events on different drives with some highlight-clips in each, but you want all of that in one event to edit from there or to bring along only those "best of" shots?
We just had that issue and tried a few things. One works ;-)
We tried to simply drag the keyword-collections to a new event. You can do that, but all you do is create a new keyword in the second event. No clips follow.
When you try to merge events you lose your smart-collections. Not good.
When you move clips to another event in the same drive.....the clip changes its reference to a new event and is gone from the original event! BEWARE!
So here´s what you do: set up an event "Highlightshow" on a drive. In your events you might have a keyword "best of event A/B/C" (my keyword here is "-aaaatest").
Select all clips in it and drag them onto the event "Highlightshow". Do this with the wanted clips from all your events.
FCPX will now COPY all clips with all their keywords and green favourite-bars to the new event/drive.
You can of course immediately edit away, because the copying happens in the background.
Now some might ask what with events on that drive. Well, events can happily live next to each other. So If you have 4 events on 4 drives and use one of those drives as the one for the event "Highlightshow", you simply drag the clips out of both events from that drive. There is no other way without messing with the original event.