This is not easy! Let's just make sure that's understood up front. :)
A SharePoint form library doesn't store flat data (e.g. a row in a repeating table). The form library stores XML forms, so what you're trying to achieve is submit an individual XML form per row, and do this iteratively for however many rows have been entered.
You may be able to:
1. Create a secondary data source with the XML schema you wish to submit to the form library.
2. In the form, iterate to set the values in this secondary data source, and submit each row into the form library.
This will most likely require writing custom code. We have a product called qRules which might be able to help, but this is a scenario that I've never seen implemented.
It might be worth taking a step back and asking why the iterative submit is needed. Will users be opening up each XML file to make changes (perhaps to other fields) and resubmit? If so, then I understand your scenario. You're basically bulk generating XML documents ahead of time for users' convenience.
Or is the data just needed in a flat structure? If so, then why not submit to a SharePoint list? A SharePoint list will store flat data and not XML documents.