Hello Dennis -
Yes, you can edit a GeoEvent Definition by editing the XML you export from Manager, but this isn't exactly recommended. The 10.2.1 product release will include the ability to edit hierarchical (group) elements as well as cardinal (list) elements. The attached ZIP contains an MP4 video which shows this. The 10.2.1 product is scheduled for public release next week (January 7th).
If you do find yourself needing to edit and/or create an event definition outside of Manager, I've included a screenshot in the ZIP which shows how the XML nodes are nested for the 10.2.1 release. I don't think this has changed since 10.2 - but that is part of the reason for recommending that you not edit the XML yourself. If the development team decided to change the node structure we would consider that an internal change not necessarily advertise it. I wouldn't want you to borrow trouble by editing a file we didn't intend you to edit. Proceed with caution.
- RJ