Beste antwoord door Lodan
Unfortunately it isn't possible to aquire the IPA file to install without a Dutch iTunes account. (Except Jailbreaks, which I do not cover after this).
T-Mobile NL doesn't see the necessity to make the app available in other counties AppStores. Mainly because the contract for T-Mobile requires a Dutch address and shouldn't be used for no longer than 4 months outside the Netherlands. This to prevent persons buying cheaper subscriptions and using it in their more expensive countries.
Best what you can do is create a new iTunes account without payment details, install the app and switch back to the US app-store. Other than that, if you have a friend with a Dutch iTunes account, you might "lend" their accounts (and install from their accounts). They even could inform you if there's an update on the app so you can take time to get the update.