Yes, you can define your own storage type. I tend to create custom ones that wrap the JSON storage type, that way I get entry point to the forms data yet still let Liferay do the heavy lifting when it comes to serialization and persistence.
After your custom storage type is deployed, when you are creating a new form if you use the peapod in the upper-right corner you can get to a configuration area where you choose the storage type. Note that this can only be set I believe when the form is first created and not afterwards, especially not after a form has been initially submitted.
