What I personally do is to add to all my JSONModel instances a method called:
-(id)mergeWithContext:
Whenever I get a JSON object from the web, JSONModel parses it for me and converts the data to what I need, then if I want to save it to CoreData I just call mergeWithContext: and pass the current context to it.
The further in my mergeWithContext: method I just create a new entity matching the current JSONModel object and copy over all values. (I actually also check whether an entity with the model's ID already exists in CoreData - then I update it, otherwise I create a new instance).
Not too difficult and you get a fair amount of flexibility if you need to add some custom behaviour when the data is saved.
mergeWithContext: returns of course the entity itself, so I can work with it further if I need to.