I took a look at the base recently, I'll explain how to add models using on the method on this client (of course there are many other ways though).
1. Open the cache folder and make a new folder called "models"
2. Place your model files into that folder, make sure the files are gzipped.
3. Login, open the command panel and type repackmodels
4. You should see something like this on your console, now just close your client.
5. Add the definitions to ItemDefinition.java (client) and item.txt (source)
Note: You can also do this before step 1
6. Login and spawn your item.
Results: Brand new models
Once you're done with adding models, you can just remove the model folder from your cache as the models are already packed.