This is another reason to look forward to the launch of the current accounts 
Tom mentioned at the 2nd to last open office that Monzo will be partnering with a provider for international money transfers when the current accounts launch.
Since Transferwise have integrated with N26 in Germany & now Starling, it seems likely that they’ll be Monzo’s partner here too 
I expect the interface in Monzo’s app will then enable us to do almost exactly what you’ve described 