This product only can be used to update the preloaded maps that came installed on your device. For example, a device preloaded with North America maps cannot be updated with a nüMaps Europe subscription, and vice versa.
Search for Garmin North America on Amazon - £63. Buy it on the micro SD card and put that in your GPS, and then you have your maps.
As has been mentioned, the device is just the device, put any maps you like in it.
Just don't download any new maps, other than your NT Europe maps, as it will overwrite anything in memory, hence easiest to add maps via SD card.
Other than buying a Garmin made SD card of America, Open Street Maps is definitely the way to go.
Do be very wary of buying ‘Garmin’ SD cards off eBay, no matter how tempting it might look. Very often the cards are keyed to a specific device, so will not work in your device, no matter how much you try.