You can use the APIs to create content and you can hotdeploy content modells via the webclient. Maybe there is even a plugin allowing for GUI-designing content models, as there is a commercial GUI plugin to generate workflows and that is related.
The easiest way would be to write your own content-model definition as XML file. For a simple case of extending the list of metadata, available, that is pretty straight forward . Check the Wiki (Browse for category) or follow the link in my signature to Jeff Potts developer series.
Norgan