NCF creatures can be activated by adding the creature pack to the data folder.
If the ID wasn't taken, everything is fine; if it was, we'll replace the existing creature. Since I haven't delved into the 5.5 files, I can't say which IDs are free.
In any case, you'll have to change two files: types.xml and Creatures.xdb.
I'll provide a detailed explanation when I have time.