[Answered ]-Django prepopulation + appending fixed string

Make the plural blank=True and do something like this in your save():

def save(self, *args, **kwargs):
    if not self.plural:
        self.plural = self.noun + 's'
    super(Noun, self).save(*args, **kwargs)

I would actually do this in javascript client side so the data entry person can correct if it is incorrect.