You can also just use ner.manual to annotate an entity type without a model in the loop. I would recommend that to start with, so that you get a feel for the data and annotate some initial examples to train a model from.
I see you've already had a response to this and maybe I'm late to the party, but once you've done some manual annotation (say 1000 examples or so) I'd retrain your model and then use ner.make_gold. I've also been doing NER with many custom categories, and it made my annotation much faster.