With the release of version 241 of Realm Works, it is no longer a requirement that you must make a copy of a predefined category in order to import into that category.
YAY TO LWD !!!
(However, note that the greyed-out placeholder text for each field will be more basic for a predefined category compared to a custom category, due to information missing from the structure file: specifically the purpose child elements, and soon-to-be description elements too).