There is (at least) one service that allows you to convert USD, GBP and EUR at the interbank spot rate, and make purchases using a prepaid MasterCard in many more currencies (also at the interbank rate). They currently don't charge any fees (as of September 2015).
You could use your US prepaid card to fund your account with Revolut and then spend them in your local currency (HRK?) without fees (you can check the current USD/HRK rate with their currency calculator); you can also withdraw to non-EUR SEPA-enabled bank accounts, but then your bank would charge you for the necessary currency conversion (both by fees and their exchange rate).
If you have a bank account in EUR, you could alternatively convert your USD balance to EUR and then withdraw that to your EUR bank account.
If your US prepaid card has a corresponding bank account which can be used for ACH direct debit or domestic wire transfers (ask the issuer if you are unsure), TransferWise or a similar service might also be an option; they allow you to fund a transaction using one of those methods and then credit an account in