Hello @smileroom
You can place the code for your preprocessor at https://github.com/AIcrowd/neurips2020-procgen-starter-kit/blob/master/preprocessors/ and add the newly added custom preprocessor to the list of available preprocessors at https://github.com/AIcrowd/neurips2020-procgen-starter-kit/blob/master/preprocessors/__init__.py.
Then you should add it to the experiment YAML file, for example,
I hope the last part is what you were looking for.