If we are not using acs-commons-nested="JSON_STORE" to store multifield data as json, need to write our own implementation like acs or we need to expose the content as json. Hope the below article would be helpful for your requirement.
If you don't want to install ACS then you would need a utility to convert node values to JSON similar to what ACS is doing. On your local only for writing logic, install ACS, debug files under this folder for multifield store as JSON: /apps/acs-commons/touchui-widgets/composite-multifield to get more details and implement in your project.
