I'm unsure what you're currently doing Edward or trying to do.
If you have a product feed in a Google Spreadsheet, then you don't need to use the shopping app to push API data into the Merchant Center. You'd just be doubling up if you have the same products in each.
Oh, so you're trying to transition over to the Google Shopping app from a spreadsheet and are looking for a way to make this transition quick? There is no way I know of. I suggest you do a few products first and see if the app serves your purpose. It has limitations that I avoid it for most AdWords clients, but it can work for some.