Add Instance Classes To Existing Model Zoo?

[cjekal]

I would like to train a model for instance segmentation to detect instances that haven't been trained on yet (e.g. like the balloon example on the Detectron2 Tutorial colab). The one difference between the balloon example and what I'd like to accomplish is that I want to add to the existing thing_classes from the model zoo. Is this feasible?

For example, in this image I've attached below, the balloons are nicely identified, but the people, bottles, chairs, etc. are not.