I just tried to extend an EnumType and discovered that it is only possible when the super type is marked as abstract.
In my given case I would've liked to only extend my EnumType with attributes where needed and keep the super type usable with seperated contents where I don't need the additional attributes.
I probably learned sometime ago why this is, but can't remeber or find the information at the moment.
The idea is that any non-abstract enum knows all its values. Values of your extension would be unknown to the extended enum.