Can I create multi-currency feeds using a 3rd party currency converter app other than the mighty Shopify Payments?

Yes, as long as you can synchronize the exchange rate that is used by the 3rd party currency converter app with the app. Currently the app supports:

• Fix conversion rate
• Shopify rate, which is updated and published by Shopify. Please note that this generally will not produce the same value as Shopify Payments, which takes into account the conversion fee.

However, if you are creating feeds for Google Shopping, be aware of the 2 constraints by Google:

• The custom currency needs to be pre-selected when the customer clicks on the ads (or when the Google bot follows the product link). This means the product link has to be currency aware (e.g. by accepting a currency parameter, like https://myshop.com/products/cool-t-shirt?currency=GBP).
• When following the product link with currency pre-selected as above-mentioned, Google bots need to read the same price value & currency from the structured data in the product landing page compared to what in the feed. Think of structured data like data under the surface of the product page. You need to make sure that your currency converter app not only shows converted price on the surface to end users, but also updates the structured data under the surface for the bots to read.
• Although not mandatory, Google generally requires a consistent price value & currency through out the whole process from when the user sees the ads, clicks on it, arrives at the product page, adds to cart then checkout.