Unfortunately, given the flexibility of Keras model subclasses, NengoDL does not provide a built in converter to convert these models. Thus, you will need to write your own converter to use with your custom Keras models.
However, our recommendation is to convert your model subclass into a functional model, and to write a custom Keras layer (and NengoDL converter only for that layer) for any custom logic you have in your call method. This approach should be less complicated than writing a converter for your entire model.