Import/transform data from one application to another and preserve record ownership

I am upgrading an application, and for various reasons it has been necessary to do so by creating a parallel application with the new forms/tables, reports, pages and permissions. I now need to move data from the old application into the new one. I created some export-specific reports that exposed all the field, including system. I now have local copies of the data in JSON files that I will be able to process and edit to match the new forms.

I do however have several questions:

I see the "added_user" field appears to be {first name}{last name}, however the export of the portal users is just an email address. How do I import the records so that the ownership is properly connected to the correct account?
How do I export and import binary data? e.g. uploaded images
Alternatively, it appears as though I can create some functions to expose the data across applications. Is it possible to execute a script so that it runs under the context of a portal user, without having to log in as that portal user?

Basically I need a way to do ETL with my data, but it seems that the only tool Zoho offers in an upload tool that works with Analytics. I'm in Creator. The only option there seems to be Create a Form by Importing Data | Zoho Creator Help, which I don't want to create tables/forms. I've already got them created.