Go to
Admin > Configuration > Localisation > Currency and then click the
Insert button to add your new currency. If you want your new currency to be the default, make sure the value is set to
1.00. Then go to
Admin > Configuration > Setting and set your new currency as the default.
Then go back and change the values of all the other currencies to convert from your default. Just go to
http://www.xe.com/ucc/ to get the correct values.