Hi,
The dialog conversion tool helps you to convert dialog from classic or/to coral2 to coarl3 UI.
It won't help you to recover content.
To get data populated when converting of Coarl2 multifield to Coarl3 multifield, you must convert JSON into nodes.
With the help of JCR API, you can parse JSON and create nodes where ever your content is used inside /content/site.
Arun Patidar