New tags must be added to the "definition" file for the game system. For Magic, the file is named "magic.def". In this file, you'll find assorted "group" elements that define groups of related tags. You can add new entries to the "edition" and "block" groups for the new release and any new block format that has been introduced. Once the entries are added to the definition file, they will be recognized and made available within CVEdit.
The only gotcha is that you need to add these entries via a text editor. The good news is that they are a single line apiece and there are numerous other entries to follow as an example.
Hope this helps!