How to save JSON with a node
Text fields can store JSON data since JSON, by definition, is a serialized format. This is provided the JSON data fits the size of your text field. If you go this route, you should make sure your field is validated for valid JSON before saving and is under your storage limits.

If you want something more versatile, you might want to add your own custom field with the Field API. There's also the JSON Field module, but it only has a D8 release.