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 support 

• 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 need to be pre-selected when the customer click on the ads (or when the Google bot follow 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 pages. You need to make sure that your currency converter app not only show converted price on the surface to end users, but also update the structured data under the surface for the bots to read.
• Although not mandatory, Google generally require a consistent price value & currency through out the whole process from when the user see the ads, clicked on it, arrived at the product page, add to cart then checkout.