Writing a new feature can be done by adding a method in fpsd/features.py.
A new classifier can be added by adding a new stanza in the Experiment._get_model_object() method in classify.py. This method must return an object that has the fit() - model training - and predict_proba() - predict scores on test set - methods defined on it. It expects these methods because that is what scikit-learn uses for its classifier objects.
To get the code to use the classifier, add a string corresponding to its name to your attack YAML file under models and add any hyperparameters and options (e.g. number of rounds for Wa-kNN) under parameters.