Custom Members Area collection user mapping

I followed all of the steps in Velo Tutorial: Building Your Own Members Area | Help Center | Wix.com and I have the custom member’s area working. The issue I am having is I have existing data for members but they are not linked to the user’s accounts. For example, I have a record in the collection has an email of testuser@email.com. If I log in to the members area, a new record gets created with testuser@email.com. I know that this happens because the _id hidden field is different than what is already in the collection.

My question is, what is the easiest way to merge the 2 records together? I ask because the existing records are tied to another collection of results, so I would need the newly created record to be tied to those same results, or have something that ties the newly logged in user to the existing record in the collection.

Does anyone know of a way I can make this work? Obviously I cannot changed the _id field since it is reserved. Is there a way I can have it check the email field, see that one exists, and use the current existing ID instead of generating a new one?