Generating a default value with the Google App Engine Bulkloader
Defining a custom conversion function, as you did, is the correct method. You don't have to modify transform.py, though - put the function in a file in your own app, and import it in the yaml file's python_preamble.
I have successfully used the bulkloader with my project before, but I recently added a new field to timestamp when the record was modified.  This new field is giving me trouble, though, because it's defaulting to null.  Short of manually inserting the timestamp in the csv before importing it, is there a way I can insert the current right data?  I assume I need to look toward the import_transform line, but I know nothing of Python (my app is in Java).
Ideally, I'd like to insert the current timestamp (milliseconds since epoch) automatically.  If that's non-trivial, maybe set the value statically in the transform statement before running the import.  Thanks.