This PR originally had info about this, but it was requested out. silverstripe/developer-docs#345
DataObject subclasses' db, has_one, has_many, etc... properties cannot be overridden in DataExtensions.
Instead, you need to use yml to override these.
---
Name: override-db-properties
---
Vendor\Package\DataObjectSubclass:
db:
SomeProperty: "Enum('original_value1,original_value2,new_value_added_here','original_value1')"