As far as i know, There is a no way to configure Import/Export json mapper dynamically in mendix.
How about use javascript action?
For example, make a entity whose attributes are {id: String, name: String, data: String}.
After that you can easily add, remove and change the value of data attribute by using JSON.parse int the javascript action.
I hope my opinion helps you.
I have encountered a similar problem and wrote a little blog about it detailing some ways to approach this.