In Magento it is possible to use multiple currencies in the site.
To do this log in to magento and go to:
System > Configuration
Under General click on currency set up.
It is possible to set the base and default display to different currencies from there drop downs.
To allow multiple currencies shift select all that are required and the save.
To set up the conversion rates you will need to go to:
System > Manage currency > Rates
Here you can manually add the different rates to each currency or import them using a web service if you have one installed and set up.
Once this is all set up a currency drop down will appear in the header of the website, where you can change the currency.