Hi NShouse,
You will need to create a service instance for it and then generate a smartobject off of it in order to connect to it. And then you can follow the document that you have provided in your post.
Heres how you may create a service instance for external sources which in this case is Currency conversion:
https://help.k2.com/onlinehelp/K2Five/UserGuide/5.1/default.htm#Tutorials/K2ForSP/ExpenseClaim/02Data/03CreateServiceInstances.htm%3FTocPath%3DTutorials%7CK2%2520for%2520SharePoint%2520Tutorials%7CExpense%2520Claim%2520Application%7CData%7C_____1
I think this one is for training purposes and connects to K2Learning servers, but it will give you an idea. You can use any services (such as google finance api) to request APIs for currency conversions.
I hope its helpful.
Regards,
Katleho