HikaShop comes with a Currency Switcher module, which you should find in the Joomla Module Manager. If it's not there for some reason, you can install and publish it just like any other module, e.g. to your sidebar.
In the HikaShop backend, under -> System -> Currencies, you can then select currencies as "published" you want to sell/invoice in, and as "displayed" you want your customers to only see.
