Yes, you can with loaddata:
python3 -m django loaddata path/to/fixture/some_new_data.json
BUT:
- don't call your new data
initial_data.json
else it will be reloaded every time you syncdb or run a South migraton and will wipe out any changes made since the fixture was loaded. (As mentioned mid-way through this page) - related to the above, loading data via fixtures can be a brittle way to do things unless you're sure that you new data won't arrive while you're writing your fixture. If it does, you risk a clash and either a failure or 'incorrect' data in your DB.
Depending on the nature of your data, it may be better to use a data migration in South. A data migration might be good when you're setting just a couple of values for lots of records (eg, changing the default of a field, or filling in an empty field with data). A data migration also lets you apply some checks/logic to things, via Python, so that you can target which records you update, or update different records in different ways, rather than writing lots of fixtures. If, however, you want to load in a ton of new records, a fixture would make more sense.