You can easily extend MSMBuilder by subclassing BaseEstimator or any of its children. You can even build your plugin to work with the msmb command-line interface.
from msmbuilder.commands.featurizer import FeaturizerCommand
class MyNiftyFeaturizerCommand(FeaturizerCommand):
    klass = MyNiftyFeaturizer
    _concrete = True
setup(
    ...
    entry_points={'msmbuilder.commands':
                       'niftyfeat = niftyfeat:MyNiftyFeaturizerCommand'
)
See the setuptools documentation for more information.