Importing data in bulk, into existing members

I have successfully followed Wix's guide to import data in bulk by converting csv to json

https://support.wix.com/en/article/how-to-import-and-export-collection-data-using-the-wix-data-api#code

However, this only adds new entries. What I would like to do is add data to existing entries and in mass. I know that this is possible using a form and entering data into each field separately, but is it possible to do this in a way similar to importing data with JSON, that lets you input data to already existing rows in your database.

Thanks