If you are looking for the decompiler itself, visit https://github.com/jindrapetrik/jpexs-decompiler
List of issues
: The issue tracker is now writable again and logged users can download files too. But note that our support is very LIMITED.
#1410 ability of adding new tags in the SystemClass of a SWF
As far as I know, it's possible to add new animations in Sprites and new sounds in Sounds, but they are becoming useless if they cannot be named. And there comes the SystemClass tags that show exactly the names of these.
You can edit SymbolClass tag - you find it in others section. Just press edit, and then add new item on the end.