If you were using an infopath form that was connected to a form library, this means that the data was being stored as XML. As such, it had access to xml features, like repeating sections and such. There is no equivalent in poweapps. Options:
Redesign the form to store data in multiple lists instead of a single list. So if the form was for customers and the repeating section was for orders, create those two lists, and connect the power app to both.
Redesign the form and store the mutiple rows as multiple lines in a multi-line text field (yes, this option is limiting. If the repeating section has a single field, then this could work well. Otherwise, see the first option)
Same idea, but instead of sharepoint lists, use either the CDS or a sql database instead of sharepoint. Pros include better performance. Cons include that licensing to use these features requires giant piles of money.
Sorry, there's really no migration path from InfoPath to Power Apps.
Or, perhaps I misunderstood your question. If you already have created a power app that uses multiple lists for the repeating sections, then all you need is a little powershell to connect to the existing forms, which again are just xml files, and then write that data to your new lists.